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ILLUMINATING ENGINEERING 


DELIVERED AT THE 
JOHNS HOPKINS UNIVERSITY 


October and November, 1910 


UNDER THE JOINT AUSPICES OF 


THE UNIVERSITY AND THE ILLUMINATING 
ENGINEERING SOCIETY 


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PREFACE 


This Course of Lectures on Illuminating Engineering was given 
at the Johns Hopkins University, Baltimore, between the dates 
October 26 and November 8, 1910, under the joint auspices of the 
University and the Illuminating Engineering Society. The origin 
and objects of the lectures are clearly stated in the preliminary 
announcement of the course, from which the following quotation 
is made: 

“The Illuminating Engineering Society recognizing the fact 
that there is an increasing demand for trained illuminating engi- 
neers, and that the present facilities available for the specialized 
instruction required are inadequate, determined, through an act 
of the Council of the Society, to encourage the establishment of a 
course of lectures on the subject of illuminating engineering. This 
course should have three objects: (1) to indicate the proper codrdi- 
nation of those arts and sciences which constitute illuminating engi- 
neering; (2) to furnish a condensed outline of study suitable for 
elaboration into an undergraduate course for introduction into the 
curricula of undergraduate technical schools; and (3) to give 
practising engineers an opportunity to obtain a conception of the 
science of illuminating engineering as a whole. 

“Inasmuch as such a course is most appropriately given at a 
university where graduate instruction is emphasized, and as the 
Johns Hopkins University has regularly offered courses by non- 
resident lecturers as part of its system of instruction, and is now 
preparing to extend its graduate work into applied sciences and 
engineering, an arrangement has been effected by which the lectures 
will be given at this University under the joint auspices of the 
University and the Illuminating Engineering Society. The sub- 
jects and scope of the lectures have been proposed by the Society 
and approved by the University. The lecturers have been invited 
by the University upon the advice of the Society.” 

The lectures were attended by 240 men from various parts of 
the United States, many of them representatives of technical 
schools, gas and electric central stations, and manufacturing com- 


vl | PREFACE 


panies. A large number of the attendants at the lectures also 
followed the course of laboratory work which had been arranged. 
The general interest in the course encourages the hope that these 
published volumes may serve to advance our knowledge of this new 
and important branch of engineering. 


II. 


VII. 


IV. 


VI. 


VII. 


VIII. 


GENERAL CONTENTS 
VOLUME I 


LECTURES 


. THE PHYSICAL BASIS,OF THE PRODUCTION oF LiautT. Three 


et eee MIEN ie aoa cee Akad ab hoe hes Mee ne oh totem Bbiehaa are 8 oot 
JosEPpH S. AMES, PH. D., Professor of Physics and Direc- 
tor of the Physical Laboratory, The Johns Hopkins 
University. 


THE PHYSICAL CHARACTERISTICS OF LUMINOUS SourcES. TJ'wo 

eeu ae ee Ohare Foie. ot she a Pgh hile Gels mcs bi aiagiaese wine ee ye ea 
Epwarp P. Hypr, PH. D., President, Illuminating En- 
gineering Society; Director of Physical Laboratory, 
National Electric Lamp Association. 


THE CHEMISTRY OF LUMINOUS SouRcES. One léecture...... 
Wiuis R. Wuitney, Pu. D., Director of Research Labo- 
ratory, General Electric Co.; Past President, American 

. Chemical Society. 


ELECTRIC ILLUMINANTS, Two le€Ctures........ccccceeeeees 
CHARLES P. STEINMETZ, PH.D., Consulting Engineer, 
General Electric Co.; Professor of Electrical Engineer- 
ing, Union University. 


(1) Gas AND Om ILLUMINANTS, (2) INCANDESCENT GAS 
ame rr meee F400 LECTULES Joc tere ov ec ca 0 esa wo ROMER Ge © ea 
_ (1) ALEXANDER C. HuMPHREYS, M.E., Hon. Sc. D., Presi- 
_ dent of Stevens Institute of Technology; Past President 
American Gas Institute. 
(2) M. C. WHITAKER, B. S., M. S., Professor of Industrial 
Chemistry, Columbia University. 


PAGE 


93 


109 


157 


THE GENERATION AND DISTRIBUTION OF ELECTRICITY WITH . 


SPECIAL REFERENCE TO LIGHTING. Jwo léctures........... 
JOHN B. WHITEHEAD, PH. D., Professor of Applied Elec- 
tricity, The Johns Hopkins University. 


THE MANUFACTURE AND DISTRIBUTION OF acerca GAS 
WITH SPECIAL REFERENCE TO LIGHTING. Two lectures..... 
(1) Mr. E. G. Cownpery, Vice-President, Peoples Gas 
Light and Coke Company, Chicago, III. 
(2) Mr. Water R. Appicks, Vice-President, Consolidated 
Gas Co., New York. 


PHOTOMETRIC UNITS AND STANDARDS. One lecturé......... 
Epwarp B. Rosa, PH. D., Physicist, National Bureau of 
Standards. 


237 


277 


387 


Vili GENERAL CONTENTS 


IX. THr MEASUREMENT OF LIGHT. TZ'wo léectures......-..e000. 
CuLaytTon H. SHarp, Pu. D., Test Officer, Electrical Test- 
ing Laboratory, New York City; Past President, Illumi- 
nating Engineering Society. 


X. THE ARCHITECTURAL ASPECTS OF ILLUMINATING ENGINEER- 

ING. Ome TECtUTE oo Ss Se Re iin oa 3 5 ace we cue 

WALTER Cook, A. M., Vice-President, American Institute 

of Architects; Past President, Society of Beaux Arts 
Architects. 


VoLuME II 


LECTURES 


XI. THE PHYSIOLOGICAL ASPECTS OF ILLUMINATING ENGINEERING. 
PUO BOCTUPES: GeO vino a iaac SS ede hel a chal a ayers nee = eee 
P. W. Coss, B.S., M. D., Physiologist, Physical Labora- 

tory, National Electric Lamp Association. 


XII. THE PSYCHOLOGICAL ASPECTS OF ILLUMINATING ENGINEERING. 

One TECTUTE oe os oa eis one eb nies she ae 4,8 fe no cee ee 

ROBERT M. YERKES, PH. D., Assistant Professor of Psy- 
chology, Harvard University. , 


XIII. THE PRINCIPLES AND DESIGN OF INTERIOR ILLUMINATION. Six 
PECTULES. oo. ae oe 6 sie e se 010 ws. 0! sue vm 6.6 ape ieee tela leaiens gent 

(1) W. E. Barrows, Jr., Assistant Professor Electrical 
Engineering, Armour Institute of Technology, Chicago, 
Illinois. 
(2) L. B. Marks, B.S., M.M.E., Consulting Engineer, 
New York City; Past President, Illuminating Engineer- 
ing Society. 
(3) Mr. NorMAN Macsetu, Illuminating Engineer, The 
Welsbach Co. 


XIV. THE PRINCIPLES AND DESIGN OF EXTERIOR ILLUMINATION. 
TREE, LECTUTES 5. « pixce o.e-4 ¢ suve 4.0 gies le pile apaitec anata 
(1) Louis BELL, PH. D., Consulting Engineer, Boston, 
Mass.; Past President, Illuminating Engineering Society. 
(2) E. N. WricutTineton, A.B., Boston Consolidated 
Gas Co. 


XV. SHADES, REFLECTORS AND DirruSING MeEpIA. One lecture... 
VAN RENSSELAER LANSINGH, B.S., General Manager 
Holophane Co. 


XVI. LIGHTING“ FIXTURES. “One! lectures. 075 32% oe ee 
Mr. Epwarp F. CALDWELL, Senior Member of Firm and 
Designer, Edward F.. Caldwell & Co., New York. 


507 


525 


575 


605 


795 


931 


XVII. 


AOVIEL 


Lists of experiments in connection with the Lecture Course, to- 
together with the necessary bibliographies 


CHARLES O. Bonp, Manager of Photometric Laboratory, United 


HERBERT E. Ives, Ph. D., Physicist, Physical Laboratory, 


GENERAL CONTENTS 


THE COMMERCIAL ASPECTS OF ELECTRIC LIGHTING. One 

SN tO gE eee eg GAME, Ghee yienas «dona e ohh im, soanle cceke a ders « is 
JOHN W. Lies, JR., M.E., Third Vice-President, New 
York Edison Co.; Past President, American Institute of 
Electrical Engineers. 


THE COMMERCIAL ASPECT OF GAS BUSINESS WITH SPECIAL 

REFERENCE TO GAS LIGHTING. One léecturé.........2scc0e: 
WALTON CLARK, M.E., President of The Franklin Insti- 
tute, Philadelphia; Third Vice-President, United Gas 
Improvement Co., Philadelphia. 


LABORATORY EXERCISES 


Gas Improvement Co., Philadelphia. 


National Electric Lamp Association. 


Preston S. Mituar, Electrical Testing Laboratory, New York. 


A. H. Prunp, Ph. D., Associate in Physics, The Johns Hopkins 


University. 


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THE PHYSICAL BASIS OF THE PRODUCTION OF 
LIGHT * 


By JoszepH S. AMES 


CONTENTS 
LECTURE I 
Physical Quantities and Measurements 


Objects and general principles of physics. 
Methods of assigning numbers to physical quantities. 
a. Measurement in terms of units. 
b. Indirect means, e. g., temperature. 
Simple ideas. 
a. Intuitive: space and time. 
b. Experimental: e. g., force (illustrated by properties of matter), 
Units of length, of time, of force; C. G. S.; English. 
Derived mechanical quantities, and their units; e. g., density, pressure. 
Measurement of length, volume, time, force, pressure. 
Errors of instruments. 
Discussion of observations. 
Definition of electrical quantities, and their units. 
Measurement of electrical quantities by portable instruments. 


LEcTURE II 
Energy and Thermal Phenomena 


Definition of work and energy: mechanical illustrations. 
Our temperature sense. Thermal phenomena. 

Thermal effects. ; 

Methods of producing these effects. 

Explanation in terms of energy. 
Meaning of ‘“ Conservation of Energy.” 

Illustrations: battery, dynamo, etc. 
Discussion of temperature and its “ measurement.” 
Discussion of modes of producing heat-effects: flames, friction, conduc- 

tion, radiation, ete. y 
Radiation and absorption: Kirchhoft’s law, “ Black Body.” 
Measurement of energy. 

a. Rise in temperature. 

b. Mechanical means. 

c. Electrical method: Bit. 





* The lectures are based upon the author’s text-book ‘“ General 
Physics,” published by the American Book Co., New York. 


2 ILLUMINATING ENGINEERING 


LEcTURE III 
Radiation 
Spectra of radiation. 
Dispersive apparatus. 
Detecting and measuring apparatus. 
Visible, ultra-violet, infra-red radiation. 
Continuous, discontinuous and absorption spectra. 
Modes of producing radiation. 
a. “ Temperature-radiation.” 
b. Luminescence: fluorescence, electrical discharge, ete. 
Color sensation. 
Cause of color of natural objects. 
a. Body absorption. 
b. Surface absorption. 
c. Exceptional cases. 
Extension of temperature scales by radiation methods. 


Lecture I 
Physical Quantities and Measurements 


Matter. Through our various senses, such as those of sight and 
hearing, we are constantly receiving sensations which we interpret 
objectively; i. e., we locate the cause of a sensation in a definite 
portion of space. We picture to ourselves the existence there of 
something which we call “matter”; and to a limited portion of 
space which contains matter we give the name “physical body.” 
Matter may be divided into two great classes: that which is living, 
such as plants and animals, and that which is not, such as pieces 
of wood and glass, water and air. Physics is, broadly speaking, 
the science concerned with this second division of matter, which 
may be called “ ordinary matter ” ; and phenomena occurring in con- 
nection with matter of this kind are called “ physical phenomena.” 

The scientific study of a subject involves three distinct ideas; 
the discovery, the investigation, and the explanation of phenomena. 
The first two require no discussion here; but it may be well to 
state that by the words “to explain a phenomenon” is meant to 
determine its exact connection with other phenomena, to describe 
it in terms of simpler ones, and in this manner to reduce the 
number of fundamental ideas as far as possible. 

In seeking for explanations of phenomena we assume either 
directly or indirectly, that there is a definite connection between - 
consecutive events, of such a nature that if we are able to reproduce 
exactly a definite condition, the same effect will follow regardless 


Tue Puysicat BAsiIs or THE PRopucTION oF LIGHT? a 


of the epoch of time or the location in space. We are justified in 
this belief by all of our experience and observations. 

Ether. ‘The careful study of the phenomena of light led philoso- 
phers, many years ago, to believe that there is present in space 
another medium for phenomena than that furnished by ordinary 
matter. It has become an accepted fact that throughout the vast 
regions of space, in the solar system and beyond, there is a medium 
permeating all ordinary matter and having many properties in 
common with matter and yet not identical with it. This is called 
“the ether.” In order to explain many electrical and magnetic 
phenomena, and even to describe the phenomena of radiation, it 
is necessary to assume its existence.* 

Physics. ‘The object of physics may therefore be defined to be 
the attempt to determine the exact connection between phenomena, 
both in ordinary matter and in the ether, and to express these 
relations with as few hypotheses as possible concerning the nature 
and properties of either. 

Physical Quantities. A physical quantity is one which we can 
imagine as capable of changing in amount, something to which we 
can assign a numerical value. Some quantities can be measured, 
others cannot. ‘To measure a quantity, another similar one must 
first be chosen as a standard or rnit, and then the number of 
times this is contained in the original quantity is its measure. 
Thus, a length can be measured in terms of an inch, a yard, a 
centimeter, etc., depending upon the choice of unit. It is possible 
to understand the meaning of a zero value of any measurable quan- 
tity; further, two or more measurable quantities of the same kind, 
for instance two lengths, may be added. On the other hand there 
are many physical quantities which cannot be measured; and yet 
it is possible to give them numerical values. Thus, the temperature 
of a body cannot be measured, although it is possible by measuring 
the change in volume of mercury in a thermometer to give a num- 
ber to temperature. 

Simple Quantities. To most physical quantities exact definitions 
can be given, but there are a few for which this is impossible; there 
are no simpler ideas in terms of which we can describe them. The 
question as to the exact number of these need not be discussed 
here, and in what follows the philosophy based upon Kant will be 


* One should add that a new school of philosophy exists which looks 
.at nature from a different standpoint. 


4 ILLUMINATING ENGINEERING 


accepted. According to this we divide our simple ideas into two 
classes; intuitive and experimental. The two intuitive ideas are 
those concerned with space and time. 

1. A straight line, a polygon, or a solid figure bounded by plane 
faces, together with the ideas involved in assigning numerical values 
to lengths, areas and volumes are considered intuitive. That is, 
it is impossible to define what is meant by length; and the idea of 
two equal lengths admits of no ambiguity. We can choose a unit 
length arbitrarily and then, making use of a method of super- 
position, determine the number to be given any length. The same 
general method may be applied to areas and volumes. 

2. In regard to time, we have a definite conception of what is 
meant by two equal intervals of time; certain physical phenomena 
appear to us to repeat themselves at intervals of time apparently 
equal, e. g., the vibrations of a pendulum or the balance wheel of 
a watch. ‘We have no way by which we can prove that these inter- 
vals are equal, yet there is every reason for believing that these 
motions of a pendulum and of the balance wheel of a watch are 
_ exactly periodic; for at any instant the external conditions affecting 
the motion are exactly the same, so far as we can tell, as they were 
at a definite interval of time before. In order to give a number 
to an instant of time one must choose some periodic motion such 
as just described, e. g., a certain pendulum vibrating under definite 
conditions, and some arbitrary epoch of time from which to count 
the number of vibrations; the number of vibrations between the 
epoch and the instant for which a number is desired is this number. 

Among the fundamental ideas of which we learn by means of our 
senses may be mentioned temperature, pitch of sound, and what we 
call “ force.” For instance, through our muscular sense we become 
conscious of certain definite sensations when with our hands or 
arms or bodies we perform certain experiments on matter. Thus, 
if a large stone is held in the hand we become conscious of a cer- 
tain property of matter called its “weight”; if we change the 
motion of a body by means of our arms, e. g., if we throw a ball 
or stop one in motion, we become conscious through the same chan- 
nel of a property of matter called “inertia.” It is possible, of 
course, to hold a body suspended from the earth and to set a body 
in motion or to stop it if moving, by other means than by our 
muscles; thus a weight can be suspended from a spiral spring and 
hang at rest with reference to the earth, a compressed spiral spring 


Tor PHysiIcAL BASIS OF THE PRODUCTION OF LIGHT 5 


may, as in a toy gun, produce the acceleration of a bullet, etc. 
Under all these conditions which are in their nature identical with 
those brought about by our muscles we say, in ordinary language, 
that “a force is acting on” the body; but it should be borne in 
mind that this is simply a description, nothing more. In order to 
assign a numerical value to a force one follows the natural way 
of studying the simplest cases of forces one can have, and then 
usmng definitions and methods based upon these observations. The 
discussion of this subject forms that branch of mechanics known 
as dynamics. 

The simplest mode of obtaining a unit or standard force, at least 
from the standpoint of the inhabitants of this earth, is undoubt- 
edly as follows: 1. Select arbitrarily a certain piece of matter. 
2. Suspend it from a fixed support by a cord. 3. Call the tension 
in this cord a unit force. It is easy to see how, by means of a 
pulley, it is possible to balance this force by an equal one obtained 
by suspending from the other end of the cord, passing over the 
pulley, another body which is added to gradually until there is a 
balance. Having thus obtained two equal forces one can obtain 
a force twice as great by balancing one body against the two used 
in the first experiment, etc. In this way a set of standard bodies 
may be obtained whose weights give forces equal to 1, 2, 3, 4, 5, 
ete., and then, if it is desired to give a number to an unknown 
force, this may be done by balancing it against a selection of these 
known forces. 

One can discuss in a similar manner methods of giving numbers 
to temperature, etc., and this will be done in a later lecture. 

‘Units. The science of mechanics is based upon our ideas of 
length, time and of force, and methods have been discussed showing 
how we can give numbers to all these quantities. It is seen, how- 
ever, that in each of these methods certain steps are arbitrary, and 
that the number finally obtained depends upon the nature of this 
arbitrary step. | 

a. Length. In giving a number to a length the first step is to 
select a length to which we give the number 1 (if we use the inch, 
we have one value for the length, if we use the centimeter we have 
a different value, etc.). The scientific world agrees to adopt as its 
unit of length the one-hundredth portion of the length of a certain 
platinum rod, kept in Paris, when this rod is at the temperature of 
melting ice. The length of this rod under these conditions is 


6 ILLUMINATING ENGINEERING 


called a “meter”; and one-hundredth of this length is called a 
“centimeter.” There are other unit lengths in daily use in this 
country and in England, but it is not necessary to discuss them. 

b. Time. In assigning a number to an instant of time we saw 
that it was necessary to select a “time-keeping mechanism,” such 
as a clock, and, secondly, to agree upon some definite instant from 
which to begin counting. The scientific world has agreed to adopt 
as its time-keeping instrument the earth itself as it rotates on its 
axis, and to use as the unit, in terms of which intervals of time are 
expressed, the “ mean solar second.” This quantity is the second of 
time referred to the “ mean solar day,” which is the average length 
for one year of the lengths of the solar days during that interval, 
a solar day being the interval of time between the two instants 
when the sun crosses the earth’s meridian at any point. It is known 
that solar days differ in length, but pendulums may be made whose 
periods are such that they agree exactly with the earth in its rota- 
tions at intervals a year apart, and these clocks are used ordinarily 
as time-keeping instruments. Different epochs are chosen in dif- 
ferent localities; these usually differ by one, two, etc., hours. 

e. Force. In assigning a number to a force it was seen that the 
essential step was to select an arbitrary piece of matter; and here 
the scientific world has agreed to use a certain piece of platinum 
kept in Paris. When this body is suspended and allowed to hang 
vertically there is said to be “a force” in the string equal to the 
“weight of one kilogram.” The thousandth portion of this. force 
is called the weight of “one gram.” In England and this country 
other unit forces are sometimes used, commonly what is called the 
weight of a “ pound.” 

The unit force on the “ centimeter-gram-second ” (C.G.S.) sys- 
tem, as used in all scientific laboratories, is the force required to pro- 
duce an acceleration of one centimeter per second per second in a 
piece of matter whose mass is one gram. ‘This force is called one 
“dyne.” The weight of one gram is very closely 980 dynes—it 
is not the same at all points on the earth. 

d. Pressure. From these fundamental properties—length, time 
and force—numerous other quantities are derived, one of which 
should be mentioned here: pressure. By pressure we mean the 
force per unit area, and, of course, the number we obtain for any 
pressure depends upon our selection of units of force and of area. 


THe Puysicat BAsis oF THE PRODUCTION oF LIGHT 4 


Measurements. It is necessary to say a few words in regard to 
the actual measurement of, or methods of assigning numbers to, 
the physical quantities so far discussed; but it is easily understood 
that for any satisfactory discussion of the subject reference should 
be made to some laboratory hand-book. 

a. Length. In the measurement of small lengths two methods 
are in general use; one, depending upon.the use of a screw and 
divided head, the other upon the use of a vernier. In the measure- 
ment of greater lengths special precaution must be taken against 
changes due to temperature, flexure, etc. 

b. Volume. Measurements of volume are made in one or two 
ways; if the volume to be measured has the shape of a simple geo- 
metrical figure, its linear dimensions are measured and its volume 
calculated ; if the volume is irregular, or if it is that of an inacces- 
sible space, a method is used depending upon our knowledge of 
the volume of mercury which is required to produce a definite 
weight at a definite temperature; e. g., the volume of a bulb may 
be determined by filling it with mercury, expelling the mercury, 
noting its temperature, and then weighing it. 

ce. Time. Methods of accurate measurement of time are too 
complicated to be discussed here. It is sufficient to note that there 
are several methods which give an accuracy of a minute fraction 
of a second. 

d. Force. The general method of measuring a force is, as stated 
before, to balance it against a known force, or a combination of 
such forces. It is possible to buy sets of weights, or a spiral-spring 
balance, which will give results sufficiently accurate for all purposes. 

e. Pressure. It is customary to measure pressures such as those 
of the atmosphere, of boilers, of water mains, etc., by balancing the 
pressure against a vertical column of mercury. An illustration of 
this method is furnished by the ordinary mercury barometer. Since 
this is the accepted method, the unit in terms of which pressures 
are most often expressed is that of ‘ one centimeter of mercury,” 
by which is meant the vertical pressure required to balance a column 
of mercury, at the temperature of melting ice, one centimeter in 
height, when the force of gravity is that which exists at sea-level 
at latitude 45 degrees. This is a perfectly definite unit, and its 
value is known in terms of the other units. 

Errors of Instruments and Observations. In this brief refer- 
ence to the measurements of these five quantities it is seen that 


8 ILLUMINATING ENGINEERING 


reliance must always be placed upon an instrument furnished by 
some instrument maker; e. g., a micrometer screw, a vernier scale, 
a set of weights, a clock, etc., and it should not be necessary to 
emphasize two facts in connection with these instruments. First, 
every instrument must, of course, be compared with the original 
standard, or with copies of it whose errors are known. It is for 
this purpose that in all, civilized countries Bureaus of Standards 
exist where such comparisons may be made. Thus every testing 
laboratory in America has or should have standards of length and 
of mass, whose values are known accurately in terms of the Paris 
standards. But, even granting that the testing laboratory has these 
standards, there are many errors or uncertainties inherent in the 
use of every instrument, and a thorough study must be made of 
it before it can be used for purposes of measurement. ‘Thus no 
screw has an absolutely uniform pitch, and the variations in this 
must be determined by known methods; no set of weights is ac- 
curate, and its errors must be learned; and similar statements are 
true in regard to every instrument. The first precaution therefore 
in the measurement of any quantity is to determine the true scale 
of the instrument, which is not by any means in all cases that 
assigned to it by the instrument maker, and also to learn the varia- 
tions in this scale in different parts of the instrument. 

Second, when an instrument is to be used for purposes of meas- 
urement it is not sufficient to simply make one observation, e. g., 
to observe once the reading on a micrometer of the diameter of a 
wire. It is necessary to repeat the measurement often. To begin 
with it is always possible that an error may be made in reading 
the figures on the. instrument or in recording them. Again, when 
the same measurement is repeated, the measuring instrument being 
removed and then. replaced, it is noted that as a rule a different 
reading is obtained. This does not mean that the quantity measured 
has changed or that the instrument used is defective, but simply 
that in the use of the instrument there are certain inherent errors 
which limit the accuracy to which it may be trusted, errors coming 
in part from the individual using the instrument, in part from 
the instrument itself, and in part from other causes. When a 
sufficient number of observations have been made one may calculate 
by known methods the most probable value to be attached to the 
quantity, and also learn something concerning the certainty with 
which this number may be regarded as approaching the true value. 


Tue Puysican BAsis of THE PRODUCTION oF LIGHT 9 


The confidence felt in their measurements by certain observers, and 
their entire lack of appreciation of the need of ascertaining the 
probable errors and uncertainties involved, is little short of astound- 
ing to one accustomed to ordinary laboratory methods. 7 

Electrical Quantities. It seems necessary in this, the first lecture 
of the course, to give a brief discussion of some quantities which 
will not be fully explained until later in the course. hese are the 
various electrical quantities ; and, of course, to most engineers they 
are all well known. In the history of electric currents many units 
have come to the front at different periods, and even at the present 
time the definitions are not the same in all countries. The differ- 
ences, however, are so slight as to justify us in neglecting them in 
all ordinary cases. The definitions given in what follows are those 
in terms of which practically all the measuring instruments now in 
use are calibrated. The unit of resistance—the ohm—is defined 
to be equal to the resistance of a column of mercury at zero degrees, 
of uniform cross-section, of length 106.3 cms., and having the 
weight of 14.4521 grams. (This column then has a cross-section 
of almost exactly one square millimeter.) 

The ampere—the unit of current—is defined to be such a current 
as flowing in a silver voltameter of a specified pattern deposits per 
second .001118 grams of silver. 

The volt—the unit of e.m. f.—is defined to be such a difference 
of potential as will produce, when applied to a conductor whose 
resistance is one ohm, a current of one ampere. 

One of the fundamental properties of current when flowing in a 
conductor is to develop heat in this conductor, and it is well known 
that a simple formula connects the heat developed and the electrical 
characteristics of the system. This matter wilt be discussed more 
fully in the second lecture. 

In order to. give numbers to the resistance of a conductor the 
current flowing in it and the difference of potential at any two 
points, various methods have been devised and instruments per- 
*fected. At the present time there are no instruments in common 
use in laboratories which have attained accuracy to such a remark- 
able degree as these. This is owing in large part to the epoch- 
making inventions of Siemens and Lord Kelvin in Europe, and of 
Weston in this country. Thanks to the efforts of these scientists 
we now have instruments for the measurement of volts, amperes 
and watts which are sufficiently accurate for most purposes. I may 


10 ILLUMINATING ENGINEERING 


be pardoned if I again emphasize the fact, however, that all instru- 
ments are imperfect and that uncertainty is attached to every ob- 
servation. 


Lecture II 
Energy and Thermal Phenomena 


Work and Energy. We are all familiar with the use of the 
words “ work” and “energy” in every-day language. They have 
been adopted in physics as names of certain physical quantities 
which admit of exact definition. Naturally these definitions have 
been made so as to coincide as nearly as possible with those every- 
day experiences which gave rise to the names originally. Thus, 
if a man raises a weight vertically from the ground, if he com- 
presses a spring, if he throws a base-ball, he knows that he is doing 
work. The essential ideas in all cases of work are, first, the action 
of a force, and, secondly, a displacement in the direction of this 
force. Corresponding to these ideas the numerical value of work 
is defined to be the product of these two quantities, i. e., the value 
of the force by that of the displacement in the direction of the 
force. It is easily seen that in all cases in mechanics the results 
of a force are either to overcome another force or to produce accel- 
eration (i. e., change of velocity of a piece of matter). Correspond- 
ing to these two types of forces there are two ways in which work 
may be done; first, when a force or opposition is overcome, as when 
a weight is lifted, a spring is wound up, a bow is bent, ete.; second, 
when acceleration is produced, as when a ball is thrown, a fly-wheel 
or grindstone is set in motion, etc. It is common experience that 
in all cases when work is done on a body, as when a weight is 
raised from the earth, a spring is wound, a body given accelera- 
tion, the body as a result gains the power of doing work itself. It 
is said to have gained “energy.” If the work done on the body 
has been done in overcoming an opposing force, the body .is said 
to have gained “ potential ” energy; whereas, if the work has been 
done in producing acceleration, the body is said to have gained* 
“kinetic ” energy. Potential energy is therefore always associated 
with a body in a strained or “ unnatural” condition; kinetic en- 
ergy, with motion, either translation or rotation. It is a matter 
of common experience also that in all cases of mechanical work one 
body loses energy and a second body gains it. Thus, if a bullet 
is expelled from a toy gun by means of the sudden relaxation of a 


THe Puysicat Basis or THE Propuctivn or LIgut 11 


compressed spring, the bullet gains energy and the spring loses it. 
It is easy to show that for all types of ordinary mechanical forces 
the amount of energy lost by one part of the system—namely, that 
which is doing work, is numerically equal to the energy gained by 
another portion of the system, that on which work is being done; 
and, as a consequence, therefore, the total amount of energy in the 
system remains unchanged. It was recognized many years ago 
that there were certain apparent exceptions which were associated 
with friction. Thus, if a fly-wheel in motion is disconnected from 
the driving shaft, its energy—as shown by its motion—gradually 
decreases, as it comes to rest under the action of friction. Here, 
then, is a case of an apparent disappearance of energy. It was 
noted, however, that in all cases like this there were certain heat- 
effects produced; and it has been established that there is an inti- 
mate connection between the loss of mechanical energy and the 
resulting heat-phenomena. Before stating this connection, how- 
ever, it may be well to say a few words in regard to our ideas of heat. 

Heat-Phenomena. Our attention is called to thermal phenomena 
by means of our temperature sense. We possess in certain portions 
of the surface of our bodies nerve endings which are sensitive to 
thermal changes in our environment. That is, if we expose our 
hands to sunshine or bring them near a stove in which there is a 
fire, or to a flame, we experience a definite sensation, and we say 
that we feel warm. Whereas, if we put our hands on a block of 
ice, or if we allow some volatile liquid to evaporate from them, 
we experience a different sensation and say that we feel cold. The 
first step in the scientific investigation of these phenomena must 
be taken by exposing a piece of inanimate matter, such as a rod of 
iron, to the same conditions as those under which we felt warm or 
cold. When this is done, it is found that the piece of matter 
undergoes various changes; and these are called thermal effects. 
In ordinary language we speak of a change from a condition when 
we feel cold to a condition when we feel hot as being a change 
from low “temperature” to high temperature. Experiments show 
that when the temperature of a body is changed, all of its physical 
properties, with the exception of its mass and weight, are also 
changed. We select ordinarily from these thermal effects a few 
of the most obvious and the most important for purposes of study 
and observation. Among these may be mentioned change in volume, 
change in electrical resistance, and change in state, as, for instance, 


12 ILLUMINATING HNGINEERING 


when a piece of ice melts and becomes liquid. On examination it 
is found that whenever work is done against friction, heat-effects 
are produced, and the investigations of Joule led him to believe 
that the connection between these two phenomena was an exact 
one, which could be stated by saying that the amount of heat-effect 
produced depended simply upon the amount of work done against 
friction, i. e., upon the apparent loss of energy, and upon nothing 
else, not upon the time taken for the change, nor the temperature 
of the working parts, etc. As a matter of fact, if we consider 
various cases in which heat-effects are being produced, we see that 
in them all work is being done against the smaller parts of the 
body which experiences the heat-effect, in such a manner that the 
energy of these smaller parts is altered. As a consequence of 
various experiments, but notably those of Joule, the scientific world 
has accepted the belief that, when we are dealing with friction or 
similar phenomena, there is no loss of energy, but that simply the 
portions of matter with which it becomes associated are too minute 
for observation with our eyes, and therefore we do not observe by 
this means the effect produced, but that this effect is shown to us 
through our temperature sense or by some heat-effect. This state- 
ment means that one can apply a numerical value to the heat-effects 
produced, in such a manner that if it is introduced into the total 
value of the energy of a system, this total value remains unchanged 
no matter how much friction may take place in the system. 
Conservation of Energy. ‘This constancy of a certain number 
when applied to the energy of a system, including in that the proper 
figure to take into account heat-phenomena, is an illustration of 
what is meant by the principle of the conservation of energy. ‘This 
principle was extended by Joule, Mayer and Helmholtz to include 
other phenomena than those of mechanics and heat. For instance, 
we know that, if we place some granules of zine in a test tube and 
pour sulphuric acid upon them, there is a violent evolution of gas 
and the test tube gets warm. This experiment can be described in 
terms of energy by saying that the internal energy of the molecules 
of the zinc and of the acid furnish the supply necessary for the 
formation of the new molecules and also for the production of the 
rise in temperature. ‘This experiment forms one of thousands 
coming under the head of Thermo-Chemistry, and all of these have 
- resulted in justifying the above description of the experiment in 
terms of the internal energy of the various substances. We also 


Tue PuHysicat Basis oF THE PRODUCTION oF Licgut 13 


know that, if we take a test tube containing sulphuric acid and 
insert into it a strip of zinc and a strip of some other metal like 
copper, the two being joined outside the test tube by means of 
some wire, we shall then have what we call an electric current. 
This is an illustration of a primary cell. In this particular type 
of cell the zinc dissolves in the acid, and there is an evolution of 
gas; the chemical side of the experiment is exactly the same as in 
the previous test-tube experiment just described. It is observed, 
however, that in the second experiment, that with the primary 
cell, there is practically no change in temperature of the test tube. 
This means, in general language, that the energy previously used 
in causing a change in temperature is consumed in this case in 
producing the electric current. As a matter of fact, we all know 
that, when an electric current is passing in a conductor, the tem- 
perature of the latter is raised; and, if the conservation of energy 
can be extended to the phenomena of electric currents, we would 
expect to find on investigation that the energy consumed in the 
heating of the conductor by the current is exactly the same as 
that which is not accounted for in the heating of the test tube 
where the chemical reactions are going on. Complete investigations 
on this point justify this belief. Joule performed many interesting 
experiments to see if in return for a given amount of work he 
always obtained the same heat-effect regardless of the method and 
mechanism by which the latter was caused by the former; thus, by 
means of a steam engine, it is possible to turn a paddle in water 
and one can note the rise in temperature of the water, or by means 
of the same engine one can turn a dynamo, thus producing a cur- 
rent which can be made to flow in a wire immersed in water, and 
again the final effect is the rise in temperature of water. In all 
cases like this it is found that the conservation of energy is fully 
justified. As a consequence of these and countless other experi- 
ments it has become an accepted belief that the conservation of 
energy can be extended to all phenomena of both matter and ether. 

Temperature and Thermometers. Before discussing questions of 
radiation and absorption as heat-phenomena it is necessary to say 
something in regard to temperature and the methods by which we 
are able to give a number to the temperature of a body. As we 
use the words hot and cold and speak of high temperature and 
low temperature in ordinary language, we are making use of ideas 
which come from our temperature senses, and therefore the tem- 


14 ILLUMINATING ENGINEERING 


perature of a body is.a term which refers to its relatwe hotness. 
It is easily seen that this quantity cannot be measured, i. e., we 
cannot regard otherwise than as absurd such an idea as selecting 
a unit of hotness and determining how many times it is contained 
in the hotness to which we wish to give a number. The words 
themselves are nonsense, It is, however, evident that we can choose 
such a measurable property of some body as changes when the tem- 
perature of the body changes, and make use of the measured change 
in this as a means of giving a number to the temperature itself. 
For instance, we can select arbitrarily a certain copper rod, measure 
its length under some condition which can be easily repeated, such 
as at the temperature of melting ice, again measure its length when — 
it is as another definite temperature, for instance, when it is 1m- 
mersed in steam under standard conditions, then measure its length 
at the temperature for which a number is desired. We can assign 
arbitrarily a certain number of steps or degrees to the interval 
between the temperatures of melting ice and of steam, say, 100; 
then an obvious method of giving a number to the temperature 
would be to take a proportion of 100 equal to the ratio of the change 
in length of the rod between melting ice and the unknown tempera- 
ture to the change in length between melting ice and steam, i. e., 
ae Vs 
Lioo— Jo 
which are justified by observations; namely, that the temperature of 
melting ice and of boiling water under standard conditions are the 
same at all points on the earth’s surface, and at all times (this may ~ 
be shown by proving that a body will always return to the same 
length when placed in a bath of ice and water, ete.) ; further, that 
the copper rod we have selected always attains the same length 
under the ‘same thermal conditions. It should be noted, too, that 
this scale gives the number 0 to the temperature of melting ice 
and 100 to that of boiling water. (It is clear that this method of 
giving a number to temperature is practically the same as that which 
anyone would follow if called upon to give a street number to a 
house erected at some point in a block otherwise vacant.) It can- 
not be emphasized too often that we have devised a method for 
giving a number to temperature, and that we have not in any sense 
tried to measure temperature. 

Some other observer might decide to take as his thermometer, 
or instrument for numbering temperatures, an iron rod and meas- 


t = 100 This system is based upon several assumptions 





Tue PuystcaL Basis oF THE PRODUCTION OF LIGHT 15 


ure its change in length; or a glass bulb containing mercury and 
measure the apparent change in volume of the mercury; or a glass 
bulb containing some gas and measure the change in pressure of 
the gas, its volume being kept constant; or a platinum wire and 
measure the change in its electrical resistance; and so on. One 
of these methods is as good as another; each gives consistent re- 
sults by itself; and, if several observers use instruments of the same 
kind, their readings are concordant. But the readings obtained 
for any one temperature by the use of different methods and instru- 
ments would all be different; and it is necessary for workers in 
scientific laboratories to come to an agreement as to which instru- 
ment they will use. The scientific world has agreed to adopt as 
the instrument for giving numbers to temperature the constant 
volume hydrogen thermometer. In various bureaus of standards 
throughout the world ordinary mercury thermometers may be com- 
pared with the standard instruments, so that the former may be 
used for ordinary purposes, as they are much more convenient. 

It is clear that this definition of temperature applies only through 
the range of temperature over which we can make use of the hydro- 
gen thermometer. When we come to temperatures so low or so 
high that there are serious defects in the use of the instrument, 
it is necessary to define other scales of temperature. For instance, 
at extremely low temperatures a helium thermometer may be used, 
or a platinum resistance instrument; and at high temperatures a 
scale of temperature based upon certain empirical laws of radiation 
may be adopted. In both these cases of the introduction of new 
scales of temperature the attempt is made to define them so that 
they agree with the gas temperatures at those moderately low and 
moderately high temperatures over which this gas scale can be 
used at the same time as the two new ones. In this way a certain 
continuity is obtained, but it must not be thought that we are 
extending the hydrogen-gas scale; on the contrary, we are intro- 
ducing new scales. | 

Radiation and Absorption.—In text-books on physics one finds 
a full description of methods of producing heat-effects such as 
flames, friction, etc., and also a description of the various methods 
by which in general these effects are distributed from one point to 
another, as by conduction or radiation. In this course of lectures 
special emphasis must be laid upon the radiation process. This is 
illustrated when we expose our hands to sunshine and in many 


16 ILLUMINATING ENGINEERING 


other similar ways. It is known as a result of experiments, which 
need not be discussed here, that the essential features of the process 
are: first, an emission from one body of energy in the form of 
ether disturbances, second, the absorption of this energy by another 
body. It is known further that all bodies in the universe are 
emitting this energy. As a consequence, therefore, of these two 
facts the question as to whether there will be any heat-effect pro- 
duced in'a body owing to radiation processes depends upon two 
things ; first, how much energy the body is losing; second, how much 
it is gaining. The phenomena of radiation and absorption of many 
bodies under different conditions have been carefully studied by 
many observers, and in the middle of the last century at about the 
same time a very important law was announced by Balfour Stewart 
in England, and by Kirchhoff in Germany. ‘The statement is 
ordinarily called “ Kirchhoff’s Law.” One form of it is to say 
that the radiating power and absorptive power: of a body are iden- 
tically the same in all respects at any one temperature; i. e., if a 
body under certain conditions radiates a certain type of energy 
more intensely than a second body, then the first body under the 
same condition will absorb that same type of energy more intensely 
than the second. (In the end this principle is an illustration of 
resonance.) In connection with this discussion of radiation and 
absorption Kirchhoff introduced the idea of a “ black body,” mean- 
ing by that a body which absorbs completely all radiations falling 
upon it; for, of course, in general, when radiation is incident upon 
a body part is reflected, part is transmitted, and only part is 
absorbed. : 
Temperature Radiation. When the radiation from bodies was 
more carefully studied it was found necessary to make certain limi- 
tations in the application of Kirchhoff’s law. Kirchhoff himself 
applied it only to those cases where radiation was to be considered 
simply as a heat process, not as a chemical or electrical one, and 
recent experiments appear to prove that we are justified in using 
Kirchhoff’s law only in the case of certain particular bodies under 
definite conditions. One way of defining this is to say that, if 
there is no. change in the molecular constitution of a body when it 
is radiating energy, its temperature being maintained constant, 
then it obeys Kirchhoff’s law; and the radiation from it is called 
“pure temperature radiation.” Other types of radiation will be 
discussed in: the following lecture. | 


Tuer PuHysicat BAsIs 0F THE PRODUCTION oF Liagut 17 


It follows, then, that since a “black body” is the best absorber 
spossible it is also the best radiator; i. e., at a given temperature 
it radiates more energy of any particular kind than any other radia- 
tor which obeys Kirchhoff’s law; and it also follows, therefore, that 
all “black bodies” radiate alike and obey the same laws. If we 
can secure such a body, then, we have an instrument of great im- 
portance. Kirchhoff himself showed that, if a hollow body, such 
as a cast-iron shell, be maintained at a constant temperature, the 
radiation inside the space was that which is characteristic of a 
“lack body” at the given temperature. If a small opening is 
made from without to the interior of such a shell, some radiation 
will escape; but the type of radiation inside will not be seriously 
affected ; and, since, through the opening we receive on the outside 
the random radiation which is characteristic of the interior, we 
can secure in this manner what is practically a “ black-body ” 
radiator. The various laws which have been deduced for the radia- 
tion from such a body will be discussed in the next lecture. 

Measurement of Energy and Power. So far nothing has been 
said in regard to the measurement of energy or the units in terms 
of which it is expressed. If we use the C.G.S. system of units, 
the standard of energy or its units is called the “ erg ”—i.e., the 
work done by a force of 1 dyne acting through 1 cm.—which is an 
extremely small quantity, so small that it is more customary to 
use 10° ergs as the unit. This amount is called a “Joule.” If 
we are interested not simply in the amount of energy but in the 
rate at which it is delivered, we introduce the word “power” to 
signify the energy delivered per unit of time, and if the amount 
of work is one Joule per second the power is said to be one “ watt.” 
(On the English system the unit of work is the “ foot-pound ” ; 
and the unit of power is a “horse-power,” which is defined to be 
33,000 foot-pounds per minute—this equals approximately 746 
watts. ) 

There are three standard ways of measuring energy; by rise in 
temperature, by mechanical means, by electrical methods. A few 
words should be said in regard to the first and third. By experi- 
ments performed by Joule, by Rowland and by others we know 
accurately the amount of energy required to raise the temperature 
of water; and by the experiments of Regnault and many others we 
know the ratio between the amount of energy required to. raise the 
temperature of water and that required to raise the temperature. 


18 ILLUMINATING ENGINEERING 


of other substances. Consequently, if we can observe the rise in 
temperature owing to heat-causes of any body of known character,, 
and of known weight, we know accurately the amount of energy 
supplied. Thus, if radiation falls upon a body and is totally 
absorbed, we have a means of measuring the amount of energy 
received. 

In the case of experiments with electric currents we know that 
the energy consumed per second is equal to the product of the 
electro-motive force and the current; and the units of the ampere, 
the volt and the watt are so chosen that, if the electro-motive force 
as measured in volts is multiplied by the value of the current in 
amperes, the product is the number of watts of power furnished by 
the current. It is easy to see how by having this simple means 
of determining power through the operation of the electric current, 
we can make use of it for the general measurement of energy. 


Lecture III 
Radiation 


Radiation. By radiation we mean those disturbances. in the 
ether which are being emitted by matter of all kinds and at all 
times. For a proper study of its nature we require instruments 
which analyze the radiation and which measure the quantity of © 
energy in the radiation. It was observed by Newton that when 
the radiation from a small source of light was allowed to pass 
through a prism of glass it was broken up or “ dispersed,” so that 
the white light of the sun, for instance, was divided into many 
colors, each particular color corresponding to radiation leaving the 
prism in a definite direction. This process of analysis of radiation 
by means of a prism is called “ dispersion”; and the investigations 
of Fresnel and others showed that what takes place is this; the 
prism transmits in definite directions trains of waves of definite 
wave-length; so that, whatever the nature of the incident radiation, 
that which is transmitted is distributed into regular groups, each 
group having a definite wave-length and leaving the prism in a 
definite direction. It was shown by Fraunhofer and others that 
one could secure dispersion by other means than by the use of a 
prism, as, for instance, by the use of a dispersion grating. 

The Bil ate by which the dispersion of light is atid taal is 
called a “spectroscope.” It consists essentially of three parts: 


THe PrystcaL BAsis oF THE PRODUCTION oF LiaHT 19 


narrow slit through which the light enters; a prism or grating to 
cause the dispersion; a lens or concave mirror to focus the different 
streams of radiation on a suitable screen, where the detecting 
or measuring instrument is placed. 

Spectra. When the radiation from any very hot source such as 
the sun or the carbons in an arc light is thus analyzed and spread 
out according to its wave-lengths, it is observed that only a-small 
portion affects the eye. This is called “ the visible spectrum.” We 
see a broad band of light, colored red at one end, and violet at the 
other. In between these there are different colors, each merging 
imperceptibly into its neighbors. Certain colors have definite 
names; and we often speak of red, orange, yellow, green, blue, 
indigo, violet, as being the “ colors of the spectrum”; yet we must 
remember that these colors are not isolated; the transition from 
red to violet is a gradual one. If a photographic plate is held in 
the region beyond the violet, it is affected intensely; and, if a 
thermometer is held in the region beyond the red, it shows by its 
rise in temperature that energy is falling upon it. We are thus 
accustomed to speak of the “ ultra-violet spectrum ” and the “ infra- 
red.” When the wave-lengths of the radiations causing in our 
eyes the color sensations are measured, it is found that a definite 
color is associated with a definite wave-length; and so we often 
speak of “ red-light,” etc., meaning radiation of such a wave-length 
as produces in our eyes the sensation of red, etc. The wave-length 
of the radiation in the extreme ultra-violet is the shortest of all; 
then, as the wave-lengths become longer, the blue end of the spec- 
trum is approached; as it becomes still longer, the color gradually 
changes from blue to green, to red, etc., down into the infra-red. 

Recording Instruments. It is not easy to find an instrument 
which will respond to waves of all wave-lengths, i. e., which will 
absorb them or will indicate the amount of the incident energy. 
For waves which are extremely short, much shorter than those which 
affect our sense of sight, we may use a photographic or a photo- 
electric process; through the visible spectrum we may also use a 
photographic process for the detection of the radiation, but for its 
quantitative measurement, either here or in the infra-red, we must 
use some modification of a thermometer. Various types of instru- 
ments have been devised and the problems are now fairly well un- 
derstood. The four forms of instruments in general use are: a, 
the bolometer, which is a thin strip of blackened platinum whose 


20 ILLUMINATING ENGINEERING 


change in electrical resistance produced by the radiation is meas- 
ured; b, the thermo-couple, or junction of two metals forming a 
closed circuit, whose E. M. F. as altered by the radiation is: meas- 
ured; c, the radio-micrometer, an instrument in which the thermo- 
electric current produced by the radiation flows through a .small 
circuit suspended between the poles of a magnet, and can therefore 
be measured by the deflection. produced; d, the radiometer, a modi- 
fication in Crookes’ original form of the instrument, depending 
upon the repulsion produced by incident radiation in a blackened 
disk suspended in a partial vacuum. Any one of these instruments, 
when properly calibrated, may be used to measure the energy of 
radiation. | | 
Classes of Spectra. If the spectra of solids and liquids are 
studied, it is found in almost every case that there is a continuous 
spectrum, having its maximum in a: region depending primarily 
upon the temperature of the source. On the other hand, if a gas 
is made luminous by the discharge through it of an electric current 
or by any other means, it is noted that its spectrum is discon- 
tinuous, i. e., is made.up of isolated trains of waves. When the 
light from a white-hot solid is allowed to fall upon any body such 
as a piece of glass or a tank containing some liquid, a certain 
amount of the radiation is absorbed by the body, and if the trans- 
mitted radiation is analyzed by a prism or a grating the resulting 
spectrum is called “the absorption spectrum” of the body. It is 
obvious that the nature of this spectrum depends not simply on 
the body itself but also on the character of the source. : 
Temperature Radiation. In the preceding lecture some time was 
devoted to the discussion of the conditions under which Kirchhoff’s 
law of radiation and absorption could be applied. It may be re- 
membered that these conditions were as follows: If a body is 
emitting radiation and if its temperature is maintained constant 
by suitable means, then, provided there are no permanent changes 
produced in the body, it obeys Kirchhoff’s law and. the radiation 
which it emits is called “pure temperature radiation.” ‘The im- 
portance of this discussion and definition comes from the fact that 
for bodies which are emitting such radiations it is possible by apply- 
ing certain general principles of physics to deduce theoretically 
certain relations between the temperature of the body and its radia- 
tion. Further, if the radiation from a “black body” is studied 
experimentally, certain empirical laws connecting gas, temperature 


THe PHysicAL BAsts oF THE PRODUCTION oF LiacutT 21 


and energy of radiation may be learned, and all “black bodies ” 
radiate alike. This matter will be referred to more in detail to- 
wards the end of the lecture. It is extremely difficult to obtain 
pure temperature radiation, though we can approximate closely to 
it by the use of a “ black body ” such as described in the last lecture. 

Luminescence. In general, however, when a body is emitting 
radiation there are changes going on in it even if its temperature 
is maintained constant by heating it from without; such bodies 
are said to: be “ luminescent.” ‘We have many types of luminescence 
and it may be worth while to say a few words concerning some of 
these. ‘There is what is called ‘ chemical luminescence,” which is 
illustrated by the slow oxidation of phosphorus; there is “ electro- 
luminescence” which we have when a gas is made luminous by 
an electrical discharge; there is “ fluorescence,” which is observed 
in many bodies and consists in the absorption of light of a certain 
wave-length, and in the emission of light of a different wave-length. 
The exact energy relation for the various cases of luminescence are 
not clear in all cases; nor is it possible to state any relations which 
connect the radiation with the physical properties of the source. 

‘Photometry. The most obvious property of radiation is, of 
course, its power to affect our sense of sight in case the source has 
a temperature sufficiently high, or in case it is emitting waves suf- 
ficiently short. As has been said, we associate different colors with 
different wave-lengths, and the question therefore as to our color 
sensation depends primarily upon two things; the nature of the 
radiating source and the power of our eyes to recognize color. The 
physiological action of the eye is to be discussed in later lectures ; 
and it may be sufficient to note here that the eyes of most people 
are competent to distinguish colors with great accuracy, provided 
the illumination is sufficiently intense. 

The most important matter connected with radiation is the ques- 
tion of the energy carried by the trains of waves of definite wave- 
length. This can be investigated obviously by means of a suitable 
dispersive apparatus and a sensitive recording instrument, such as 
a bolometer or radio-micrometer properly standardized. But this 
is largely of theoretical importance. What we are most closely 
concerned with is the question as to the intensity of the effect of 
radiation upon our eyes. The investigation of the various problems 
connected with this forms the science of photometry. We must 
- find suitable methods of comparing the efficiency of various sources 


22 ILLUMINATING ENGINEERING 


of light in producing light sensation; this implies a study of the 
intensity of the light sensation, of the energy required for this, 
and of that portion of the energy of the source which is radiated in 
the invisible portions of the spectrum. 

Colors of Objects. We are concerned most often, however, not 
with the color of the source of light itself but with the color which 
natural objects appear to have when viewed in a certain. light. 
‘We ordinarily call a leaf green, a brick red, ete., meaning simply 
that when viewed.in sunlight these objects have these colors. If we 
study carefully many cases of colored objects we soon recognize 
that their color is in general due to one of two causes. The com- 
monest of all causes is what is called “ body absorption,” and is 
illustrated perfectly by a piece of colored glass, a tank of colored 
water, flowers, etc. The process is as follows: The incident light 
penetrates into the body, where certain trains of waves of definite 
wave-lengths are absorbed, and where the rest of the light is either 
transmitted or is scattered in all directions by small inequalities or 
dust particles. Consequently, if one looks at the object either by 
transmitted light or from any direction, he will receive in his eye 
only that portion of the incident light which is left over after the 
absorption in the interior of the body. If the incident lght is 
white, and if red light is absorbed by the body, it will appear blue, 
because when white light loses its red constituent it becomes blue. 
It is evident therefore that the nature of the color which an object 
appears to us to have depends vitally upon the nature of the hght 
in which it is viewed, because we see in the end that light which 
is the result of subtraction from the incident light owing to ab- 
sorption. The same body will appear to us of a different color, 
if the color of the source is changed. If the light after passing 
through one colored object is allowed to fall upon a second, and if 
we view this transmitted light we have, of course, a double sub- 
traction. ‘This is the process which we have ordinarily in the mixing 
of paints. The explanation of the color of a painted object is ex- 
actly that just given; the light enters a short distance and is 
scattered out, so that if two paints are mixed we have a double 
subtraction. It is hardly necessary to emphasize the importance of 
this general discussion of color in the question of the illumination 
in a room, 1. e., the effect of the color of the wails, curtains, etc., 
upon the Pensell illumination, etc. 


CI 
THer PuHysiIcaL BASIS oF THE PRODUCTION oF LIGHT 23 


There are certain objects, however, which owe their color to a 
process different from this, as, for instance, metals and the aniline 
dyes. In their case the incident light suffers absorption at the 
surface, not in the interior, and so their color is said to be due to 
“surface absorption.” 

There are many other exceptional cases of color about which noth- 
ing need be said at the present time, such as the colors associated 
with luminescence, interference, the scattering th to fine parti- 
cles, ete. . 

Laws of Temperature Radiation. The most important type of 
radiation is, as has been said repeatedly, pure temperature radia- 
tion; and for many years many competent observers have been in- 
vestigating the connection between the temperature of the “ black 
body ” emitting such radiation and the nature of the spectrum 
and the amount of the energy. It has been shown that, if all the 
energy emitted is measured by using a suitable absorbing instru- 
ment, the connection between the temperature of a source and the 
total quantity of the energy may he expressed by an extremely 
simple formula, namely, 

energy emitted=a(t+273)4, 

where t is temperature on the gas scale, and:a is a measurable con- 
stant, independent of temperature. This is called “ Stefan’s Law.” 
This evidently furnishes a means of defining a scale of temperature 
in a region where a gas thermometer could not be used, since we 
can measure the energy emitted by bodies at all temperatures. The 
method, of course, is to take the law as given, which states the 
relation between gas temperature and energy over the extreme range 
to which a gas thermometer can be used, and define the tempera- 
ture for regions of higher temperature by the formula itself. That 
is, we would measure the energy from a certain source and by the 
use of the formula deduce the value of the temperature. It should 
be clearly understood that there is no assumption involved in this; 

it is a matter of definition. 

It has been found further that, when the energy of a “ black 
body ” has been dispersed into its spectrum, and the amounts of 
energy carried by trains of waves of definite wave-length are meas- 
ured, there is also a connection between the distribution of this 
energy as a function of the wave-length and the temperature of the 
source, as measured on the gas scale. Several formulas have been 


> +4 
24 ILLUMINATING ENGINEERING 


derived from these experiments; and here again we have a means 
of defining a temperature scale which can be applied to extremely 
high temperatures. All these scales defined by radiation formulas 
seem to agree to a high degree of accuracy. 

One of these relations, known as Planck’s law, may be written 


where E, is the energy carried by waves whose wave-lengths lie 
between A and A+dA, T is written for t+273,'e is the base of the 
natural system of logarithms, C, and C, are constants. 

Two other relations are: : 


Anie Usaconst: 


= = const. 
where T is again written for t+273; Amaw is the wave-length cor- 
responding to the maximum value of E, for the temperature T; 
and E, is the value of E, at this wave-length Amac- 





II 


THE PHYSICAL CHARACTERISTICS OF LUMINOUS 
SOURCES 


By Epwarp P. Hypsg 


CONTENTS 
LECTURE I 

1. Introduction. 

A. What is light? 

B. The conditions to be fulfilled by light sources. 

C. The sources of supply and loss of energy. 
2. Luminous efficiency. 

A. Sensibility of the eye to energy of different wave-lengths. 
a. Time relation between stimulus and sensation. 
b. Sensibility a function of absolute intensity of illumination 

(Purkinje effect). 
Luminosity curves for various illuminants. 
. Mechanical equivalent of light. 
a. Unsatisfactory nature of ordinary definition. 
b. Mechanical equivalent of most efficient monochromatic radia- 
tion (M= 800 lumens per watt). 
D. Highest possible efficiency of white light (about 300 lumens per 
watt). 
E. Highest possible efficiency of black body radiation (about 140 
lumens per watt). 

F. Quantities entering in discussion of efficiency. 

a. Power supplied to lamp (Q). 

b. Power radiated by lamp (R). 

c. Power dissipated by convection (C,). 
d. Power dissipated by conduction (Cy). 
e 
f 


on 


. Power radiated in visible spectrum (L). 
. Luminous flux in lumens (¢). 
3. Quality of light. 
A. Integral color of composite light. 
B. Spectral distribution. 
4. Temperature radiation. 
A. Black body radiation. 
a. Properties of the theoretical black body. 
b. Quantity and quality of black body radiation at various 
temperatures. 


26 


B. 


ILLUMINATING ENGINEERING 


c. Ratios of energy radiated in visible spectrum to total energy 
L 
radiated Ge at various temperatures. 
d. Ratios of luminous flux to energy radiated in visible spec- 
trum (+) at various temperatures. 
e. Ratios of luminous flux to total energy radiated (x) at 
various temperatures. 
f. Temperature of highest possible efficiency of black body 
about 6000° absolute. 
Selective radiation. 
a. No natural body is absolutely “ black.” 
1. Difference in emissivity—‘ gray ”’ bodies. 
2. Difference in spectral distribution—“ selective ’’ bodies. 
b. Gray bodies have same efficiency as black bodies at same 
temperature. 
c. Selective bodies may have higher efficiency than black body 
at same temperature. 
d. Metallic filaments as a rule owe efficiency in part to 
selectivity. 


5. Luminescence. 


A. 
B. 
C. 
De 


Accepted definition of luminescence. 
Query as to significance of term “ luminescence.” 
Employment of terms in present lectures. 
Types of luminescence. 
a. Chemi-luminescence. 
b. Photo-luminescence or phosphorescence. 
c. Electro-luminescence. 


LECTURE II 


1. Introduction. 
2. The physics of the electric incandescent lamp. 


A. 
DB: 


C. 


D. 


C’?R loss in leading-in wires. 

Loss by thermal conduction and convection of gas negligible in 
commercial lamps. 

Relation between loss through gas and pressure of gas for special 
platinum filament lamp at about 1700° absolute. 

Loss by thermal conduction along leading-in wires and anchor 
wires not more than 5% for commercial tungsten and 7% for 
tantalum lamps. 


. Radiation arises from temperature and not iuminescence. 
. Efficiency of metal filament lamp partly due to temperature of 


operation and partly to favorable selectivity. Osmium prob- 
ably most selective of ordinary filaments. 


L 
. Values of R for incandescent lamps. 


. Relations between voltage, current and candle-power for in- 


candescent lamps. 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES BY 


3. The physics of the arc lamp. 


A. 
B. 
C. 


: : L 
. Data on conduction and convection losses, and on values of =- 


Definition of “arc.” 
Characteristics of arc discharge. 
Distribution of potential in the arc. 
a. Fall of potential at anode. 
b. Fall of potential along vaporous path. 
c. Fall of potential at cathode. 


. Sources of luminous flux in the arc. 


a. Anode principal source of luminous flux in direct current 
open and enclosed arcs. 

b. The two electrodes equally the principal sources of luminous 
flux in alternating current open and enclosed arcs. 

c. The luminous vapor the principal source of luminous flux 
in “luminous” and “ flaming” arcs. 


. The difference between “luminous” and “ flaming” ares im- 


portant from physical standpoint. 


. Is luminosity of gas to be ascribed to selective temperature 


radiation or to so-called ‘“‘ luminescence ’’? 


. Probable temperatures of anode, cathode and vapor in open 


carbon arcs. 


. Conduction and convection losses in arc lamp not accurately 


known. 


L 
. Values of and ae for various types of arc lamps. 


physics of low pressure arcs and vacuum tubes. 


. Distinction between arc and vacuum tube discharge. 
. The ordinary mercury vapor lamp an enclosed luminous arc at 


low pressure. 
a. Efficiency ascribed to luminescence with large percentage of 
radiation in the visible spectrum. 


. The mercury arc in quartz tube operated at higher current 


density and increased efficiency. 
a. Temperature radiation supposed to supplement luminescence 
in quartz mercury arc. 


R 
for mercury arcs meager. 


. In vacuum tube discharge the character of the light depends on 


nature of gas between electrodes. 


. Owing to distribution of potential in vacuum tubes, long tubes 


are necessary for high luminous efficiency. 


. Luminous efficiency of vacuum tube sources ascribed to lumi- 


nescence. 
physics of open flames, and of the incandescent mantle. 


. The ordinary open flame owes its luminosity to the temperature 


of carbon particles heated to incandescence. 


. The temperature of Bunsen flame about 2100° absolute at its 


hottest part. 


28 


ILLUMINATING ENGINEERING 


. The peculiar radiating properties of rare earths and their 


mixtures. 


. Hypotheses that have been advanced to account for high effi- 


ciency of mantles. 


’ a. Luminescence. 


b. Localized high temperature due to catalysis. 
c. Selective emission at temperature consistent with that of 
Bunsen flame. 


. Most generally accepted theory at present that given under 


D—c, but question still in doubt. 


. Peculiar phenomena of mixtures of thoria and ceria explained 


on basis of relative emissivities and selectivities of the 
two substances. 


. Estimates of temperature of incandescent mantle. 


L 
. The luminous efficiency of mantle and values of se 


. Temperature of acetylene flame. 


L 
. The luminous efficiency of acetylene, and the value of a 


physics of the Nernst glower. 


. The glower a “solid electrolyte,’ composed of oxides of rare 


earths. 


. Conduction, convection and other losses. 
. Probable temperature of glower. 


if 
. The luminous efficiency of the glower and the value of “hs 


physics of the fire-fly and other light-producing organisms. 


. The high efficiency of the fire-fly due to extremely selective 


luminescent radiation. 


. Light-giving properties of bacteria and other organisms. 


distribution of energy in the spectra of the various luminous 
sources. 


. Spectra of gases, liquids and solids. 


a. Unique spectra of rare earths. 


. Energy distribution in visible spectrum of ordinary illuminants. 
. Energy distribution in infra-red spectrum of ordinary illu- 


minants, 
quality of light from the various luminous sources. 


. Integral color and continuity of visible spectrum. 
. Colorimetric measurements of ordinary illuminants. 


Lecture I 


1. Introduction 


The sensation of light is produced normally when radiant energy 
transmitted through the luminiferous ether in electro-magnetic 


waves 


of sufficient amplitude, and within certain limits of wave- 


length impinge upon the retina of the eye. It is necessary to 


$ 
PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES 29 


keep in mind that the ultimate object of every luminous source is 
to produce the sensation of light, and that therefore the relation 
between the psycho-physiological sensation and the physical stimulus 
furnishes a fundamental criterion in an analysis of the physical 
characteristics of luminous sources. 

However, the first condition to i fulfilled by a luminous source 
is that it radiate energy within the limits of the visible spectrum. 
This is the initial condition, but there are many other conditions, 
physical and non-physical, scientific and aesthetic, which determine 
the real efficiency of a luminous source, where by efficiency is meant 
the degree of adaptability to the required end. From a physical 
standpoint, the energy relations in the production of luminous 
energy are of prime importance. The interest centers in the ef- 
ficiency of the transformation of the energy supplied to the lamp 
into the light received from it. 

A definite amount of what is familiarly termed ichematdd: energy 
is stored up.in the molecules of acetylene and oxygen. After com- 
bustion a smaller amount of energy is stored up in the resultant 
molecules of CO, and water vapor, a part of the residue becoming 
available as light. The gross efficiency of the combustion of acety- 
lene as a source of light is the ratio of the light produced to the 
energy stored up in the molecules of acetylene and oxygen before 
combustion, the two being measured in appropriate units. The 
energy stored up in the resultant molecules of CO, and water vapor 
may be considered as waste so far as the present transformation 
is concerned. 

This example illustrates chemical rather than physical relations 
in transformation of energy, but serves to show that in many cases 
the two are intimately interconnected. Judged from a purely 
physical | aspect the efficiency of the acetylene lamp depends en- 
tirely upon the ratio of the light produced to the energy liberated 
in the chemical transformation. Thus some of the energy is dis- 
sipated by conduction, some by convection and some by radiation. 
Of the latter a relatively small part is available as light. The 
matters of fundamental importance to the physicist, therefore, are 
the relations of the energy dissipated by conduction and convection 
to that radiated, the spectral distribution in the radiant energy, and 
the causes which determine these relations, 

The incandescent electric lamp furnishes an interesting illustra- 
tion. A definite amount of energy per second i is supplied electrically 


30 ILLUMINATING ENGINEERING 


to the terminals of the lamp. A part of this is transformed into 
heat by the C?R loss in the leading-in wires and junctions. The 
remainder is transformed into heat by the passage of the current 
through the high-resistance filament. That which is transformed 
into heat by the C?R loss in the leading-in wires is completely lost, 
as far as its direct influence on the luminous efficiency of the lamp 
is concerned. This loss in the ordinary types of lamps manufac- 
tured at the present time is negligibly small, amounting in most 
cases to less than 1 per cent. 

The energy which is transformed into heat in the filament is 
dissipated in various ways, only a small part of it ultimately be- 
coming available for the production of light. A part of the energy 
is dissipated by conduction and convection by the gases in the bulb 
in cases where the vacuum is not high, but this loss in a good lamp 
is entirely negligible. Another portion of the energy is dissi- 
pated through heat conduction by the leading-in and anchoring 
wires. Thus, owing to the high temperature of the filament com- 
pared with that of the leading-in and supporting wires with which 
it comes into contact, there is a continual heat conduction away 
from the filament at these points, thus cooling the filament locally 
and decreasing its luminous efficiency. 

The remainder of the energy transformed in the filament is 
radiated, the spectral distribution depending upon the temperature 
of the filament. Only that portion which is radiated in waves 
within the limits of wave-length of the visible spectrum is pro- 
ductive of light. As stated above, the loss due to conduction and 
convection by the gas in a normal lamp must be neglgibly small. 
It is quite a simple matter, however, to show what a saving is 
effected in the case of an ordinary incandescent lamp through the 
use of an evacuated bulb. If a lamp is constructed having a fila- 
ment of some material, such as platinum, which can be operated 
either in air or in a vacuum, the difference in power supplied to the 
lamp when evacuated and when filled with air, the temperature 
of the filament being the same in the two eases, is quite large. — 
Thus a platinum filament of 0.1mm. diameter and 15 cm. length, 
mounted in a pear-shaped bulb of 8cm. maximum diameter and 
13 cm. length, when operated at a temperature of approximately 
1700° Abs. (Centigrade+ 273°), requires 4.75 watts when the bulb 
is evacuated, and 24.3 watts when filled with air at atmospheric 
pressure. In other words, the loss by convection and conduction 


PHYSICAL CHARACTERISTICS OF LumMINous SourRCcES 31 


of the gas is 400 per cent of the total power he ahi to operate 
the filament in a vacuum. 

The losses by conduction at the leading-in and anchoring wires 
have been variously estimated, the values found ranging from an 
almost negligible quantity to as high as 25 or 50 per cent in various 
types of standard lamps.’ Attempts at direct measurement of the 
energy radiated seem to indicate comparatively high figures for 
the thermal conduction losses, whereas the conclusion from prac- 
tical experience in lamp manufacture points to rather small losses. 
Preliminary measurements by a new direct method gave for these 
losses for normal carbon, tantalum and tungsten lamps values in 
all cases of the order of magnitude of 5 per cent, which would seem 
to be more consistent with the experience of lamp manufacturers 
than the much larger losses found by other investigators. 

If then the losses by convection and conduction amount to but a 
small percentage of the total energy supplied to the filament, ex- 
planation of the relatively low luminous efficiency of the lamp must 
be sought.in the spectral distribution of the radiated energy. 


2. Luminous Efficiency 


Of the energy radiated by a luminous source only that portion 
which lies within the wave-length limits of visibility produces the 
sensation of light. Even within these narrow limits the intensity 
of the sensation varies greatly with the wave-length when the retina 
is excited with equal quantities of energy. Thus a quantity of 
energy which in the deep red or extreme violet is scarcely sufficient 
to be visible, would in the yellow or green regions of the spectrum 
produce a moderately strong sensation. 

The extreme wave-lengths which mark the limits of the visible 
spectrum are somewhat variable, depending on the individual. For 
normal eyes radiant energy between the limits of wave-lengths of 
0.8 » (4=0.001 mm.) on the red side to a little less than 0.4 » on 
the violet side produces the sensation of light. With moderately 
intense sources the eye can perceive rays of wave-lengths down to 
0.38 », but there is no sense of color beyond 0.4 p. 

The energy contained in the visible spectrum of the radiation 
from an ordinary solid at ordinary temperatures comprises but a 
very small fraction of the total energy radiated. Beyond the visible 
on the red side, the infra-red spectrum extends from 0.8» to in- 
definitely longer wave-lengths, which have been isolated and studied 


32 ILLUMINATING ENGINEERING 


up to 96.7 uw. It is in this region that in most cases the great bulk 
of radiant energy is emitted. Thus, in the case of the tungsten 
lamp about 95 per cent of the energy radiated by the filament is 
emitted in the form of heat rays of wave-lengths too long to excite 
the human retina. | 

Beyond the visible spectrum on the violet side the ultra-violet 
spectrum extends from about 0.4 » or 0.38 » to indefinitely shorter 
wave-lengths which have been isolated and studied down to 0.1 np. 
The energy radiated in the ultra-violet region of the spectrum is 
for all ordinary sources very small, even compared with that radiated 
in the visible spectrum, and may generally be neglected in the fol- 
lowing discussion. 

It has been stated that the energy radiated in the infra-red and 
ultra-violet regions of the spectrum does not conduce to the sensa- 
tion of hght, and that even within the narrow limits of wave- 
length comprising the visible spectrum: equal quantities of energy 
in different portions of the visible spectrum do not produce the 
same intensity of sensation. It is of much interest, therefore, and 
most pertinent to the question of the efficiency of light sources, to 
consider briefly the relation between the energy of the stimulus and 
the intensity of the resultant sensation for the various wave-lengths 
lying within the limits of the visible spectrum. 

At the outset it is necessary to note that the intensity of the 
sensation does not depend solely on the intensity of the stimulus, 
even for any one wave-length. The time interval during which 
the stimulus acts determines, to some extent, the intensity of the 
sensation. There is a lower limit to the duration of the stimulus, 
below which no sensation is produced. As this time interval is 
increased the sensation rises rapidly for some wave-lengths even 
beyond that of permanent régime and then falls again to what has 
been termed the permanent régime, or normal sensation. All of 
this occurs within a fraction of a second. After the retina has 
been exposed for a long time to a constant stimulus, the sensation 
gradually decreases owing to fatigue. The element of time, there- 
fore, plays an important role in determining the intensity of sen- 
sation for a given stimulus. 

There is a second element which should be mentioned at the 
beginning as determining the relation between the intensity of 
the sensation and the intensity of the stimulus for different wave- 
lengths. If there have been found two quantities of energy in the 


PHYSICAL CHARACTERISTICS OF LuMINOUsS SouURCES 33 


red and blue ends of the visible spectrum, respectively, which pro- 
duce equivalent intensities of sensation where the absolute intensity 
of sensation is low, it does not follow that the two sensations will 
remain equivalent if the quantities of energy are greatly increased, 
even though each is increased by the same relative amount. The 
red sensation. at the higher intensity would be relatively larger. 
This phenomenon is familiarly known as the Purkinje effect, and ’ 
may be stated in general as follows: The relative intensities of 
sensation for equal energy excitation in different portions of the 
visible spectrum depend upon the absolute magnitude of the energy 
stimuli. In other words, the relation between the increase in sen- 
sation and the increase in stimulus is not the same for different 
wave-lengths in the visible spectrum. 

In addition to these two elements of interval of duration and 
absolute magnitude of the stimulus in determining the relative 
sensations produced by equal quantities of energy in the different 
portions of the visible spectrum, there are other psycho-physiological 
elements which will not even be mentioned here. Moreover, the 
two elements which have been described briefly will not be con- 
sidered further in the discussion. It will be assumed, (1) that 
in every case the stimuli act over a sufficiently long interval to 
produce the normal sensations of permanent régime; (2) that the 
absolute magnitudes of the stimuli are always moderately large, 
since it is only at relatively low intensities of illumination that 
the Purkinje effect is distinctly noticeable. 

What, then, under normal conditions, is the relation between the 
intensity of the stimulus, and the intensity of the sensation in 
different portions of thé visible spectrum? The answer is given in 
Figure. 1. 

The so-called sensibility curve which gives this relation is com- 
monly obtained by determining the quantity of energy per second 
necessary in different portions of the spectrum to produce the same 
luminosity, i. e., the same intensity of sensation. The reciprocals 
of these quantities of energy are then plotted as the sensibility 
curve. The curve obtained-in this way is shown in [Figure 1. 
Neglecting the variations caused. by the Purkinje phenomenon, the 
relative candle-powers of two sources may be computed by multi- 
plying the ordinates of the spectral energy curves of the two sources 
by the ordinates of the sensibility curve, and comparing the areas 
_ enclosed by the two luminosity curves thus obtained. 


34 ILLUMINATING ENGINEERING 


Luminosity curves obtained in this way for a number of common 
light sources are given in Figure 2. Curves a, b, c, ete., are the spec- 
tral-energy curves for the 3.1 w. p.c. carbon lamp, the 1.25 w. p. c. 
tungsten lamp, the Nernst lamp, and the Welsbach mantle (99.25° 
per cent thoria, 0.75 per cent ceria) and curves a’, b’, c’, etc., are 
the corresponding luminosity curves, i. e., the curves showing the 
relative intensities of sensation produced in different parts of the 
spectrum. The energy curves are so drawn that the total energy 
in the visible spectrum (taken arbitrarily for this particular il- 
lustration as extending between the limits of wave-length A=0.70 pu 











Vit esl) ei i el i lap tee 











. BEECHER 
| 


LT 


oe andaun 


ra | 


“RBIS S SBA SE Se 


~ 
a 


wi 
| 
bd 
|| 
wa 


\ 
ma 
| IAI 


Reciprocals of intensities of radiation 






A slestea holed et i rebate ae 
SN eee 
dX in ie 
Fiq. 1.—So-Called Sensibility Curve. 
(Luminosity Curve for Equal Energy Distribution. ) 


AGRE REGRMaGANS Se Ae Ve wise 


{ive (RSC aenees See. eome 
JERE @ 


on the red side to A=0.48 » on the violet side) is the same for all. 
In other words, the areas enclosed by the energy curves and the 
axis of abscissas, between the two limiting ordinates, are equal. 

It is seen from an inspection of the luminosity curves a’, b’, etc., 
that although the eye has its maximum sensibility at A=0.545 p, 
the wave-length of maximum luminosity for most sources is shifted 
well toward the red end of the spectrum, owing to the predominance 
of energy in the longer wave-lengths. Moreover, the wave-lengths 
of maximum luminosity for the various sources are somewhat differ- 
ent, as are also the shapes of the luminosity curves, owing to the 
different distributions of energy in the spectra of the various 
sources. | 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES 35 


The literature on the efficiency of various light sources contains 
many reports of determinations of the mechanical equivalent of 
light* where by this term is meant the energy per second within 
the limits of the visible spectrum which will produce a unit flux 
of light, measured photometrically—in other words the watts per 























































































































ECAH 























pal od 
Sa I a 
ass ae 


\ 




















cov iaas ce 
RiieRe 


d\ in p. 


Fig. 2.—Energy and Luminosity Curves for Various Light Sources. 


lumen. The determination of the mechanical equivalent of light 
is an attempt to correllate flux of energy, measured in watts, with 
the resultant sensation produced, measured in light units. As ordi- 
narily determined, however, it is subject to criticism in two re- 
spects: (1) the value found for any light source depends upon the 
wave-lengths arbitrarily chosen as limiting the visible spectrum ; 
(2) for any definitely chosen limits of wave-length, the value de- 


36 ILLUMINATING ENGINEERING 


pends on the light sources used. Both deficiencies arise funda- 
mentally from the same cause, viz., that the mechanical equivalent 
of light is different for every color or wave-length, and therefore 
has definite significance only as applied to light of some one wave- 
length. 


TABLE I 
MECHANICAL EQUIVALENTS OF LIGHT AS GIVEN FOR SEVERAL ILLUMINANTS 
(Wave-length limits taken as 0.38 uw and 0.76 “.) 


Source Authority bd heed oo 
HIGIneT <seeniks . tosis Lace ee oe cs Angstrém .0096 
ALO eG Seer tee tek ce tetera Drysdale .0064 
Nernst. rc Oe ee ee ee < .0095 
Black body at 6000° Abs......... Ives .0080 
Ideal yeilow-green light .......... ¥ .0012 


In Figure 2, curves a’, b’, etc., are luminosity curves correspond- 
ing to equal quantities of energy between the wave-lengths A=0.70 » 
and \=0.43 ». The areas enclosed by these luminosity curves and 
the axis of abscissas taken between the two limiting ordinates might 
be taken as giving the relative values for the mechanical equivalent 

of light obtained from these sources. The values thus obtained, 
~ however, are not comparable: with those usually given, because 
ordinarily the limits of wave-length are taken 0.76 » and 0.38 p. 
But although it is true that visibility extends to these wave-length 
limits, the luminosity at the two ends is so small that it might be 
neglected. On the other hand, energy at the red end between 
0.70 » and 0.76 » would constitute for most sources a large per- 
centage of the total energy in the visible spectrum. In Figure 2 
the narrow limits are taken, because for some of the sources given 
accurate energy curves to the larger limits were unobtainable. 

In Table I some values of the mechanical equivalent of hght 
for various sources, and between the customary limits of 0.76 p 
and 0.38 » are quoted. 

Although the uncertainty in the actual values given may be 
great owing to the difficulty of measuring accurately by objective 
methods the small quantity of energy in the visible spectrum, the 
differences in the relative values obtained for different light sources 
are largely due to the fact that the mechanical equivalent for every 
different light source having a different spectral-energy distribu- 
tion is necessarily different. . 


PHYSICAL CHARACTERISTICS OF LUMINOUS SoURCES 37 


A much better definition of the term “ mechanical equivalent of 
light ” would be the flux of energy (in watts) for some definite 
wave-length—preferably the wave-length of maximum sensibility 
(A=0.545 »)—that produces a unit flux of light, measured photo- 

metrically (one lumen). From the best determinations of this 
' quantity up to the present time the most probable value for 
A= 0.545 pw is of the order of magnitude of 800 lumens per watt, 
or, aS more commonly expressed, 0.015 watt per mean spherical 
candle, though these values may be in error many per cent. Hence 
the most efficient light source that could be imagined would be one 
in which all the energy supplied to the lamp is transformed into 
radiation, and all this radiant energy is concentrated in light of 
that wave-length (A=0.545 ») to which the human eye responds 
most intensely. The efficiency of this source would be 800 lumens 
per watt, or 0.015 watt per mean spherical candle. 

This ight, of the single wave-length, A=0.545 pw, would, of course, 
not be white. Its color would be yellowish-green, and it would be 
very unsuitable for ordinary illumination both on account of its 
own color and also because of the unnatural appearance objects 
illuminated by it would assume. The question naturally arises, 
“What would be the highest possible efficiency of white light, if 
all the energy supplied to the source was transformed into radiation, 
and all this radiant energy was concentrated within the limits of 
the visible spectrum in such a way as to produce white light, where 
by white light is meant a distribution of energy in the visible spec- 
trum similar to that in the spectrum of average noon-time sun- 
light?” The answer to this question is approximately 300 lumens 
per watt, or about one-third the efficiency of the most efficient mono- 
chromatic light. Expressed in watts per candle the specific con- 
sumption of the most efficient white light would be about 0.04 watt 
per mean spherical candle. If the limits of the visible spectrum 
were taken as 0.70 » and 0.43 » the corresponding figures would be 
400 lumens per watt or 0.03 watt per mean spherical candle. 

Compared with this the efficiency of those ordinary illuminants 
for which we can measure the power supplied in watts is extremely 
low. The flaming arc has an efficiency of about 50 lumens per 
watt, or 0.25 watt per mean spherical candle. The tungsten lamp 
has an efficiency of 8 lumens per watt, or approximately 1.6 watts 
per mean spherical candle (1.25 watts per mean horizontal candle). 
Expressed in another way, if the efficiency of the most efficient 


338 ILLUMINATING ENGINEERING 


monochromatic source is taken as 100 per cent, the efficiency of the 
most efficient white light is approximately 15 per cent, the efficiency 
of the flaming are and tungsten incandescent lamp are, ae ee aan 
6 per cent and 0.9 per cent. 

The reasons for the relatively low efficiencies of artificial sources . 
compared even with the most efficient white light are threefold, as 
indicated in the illustration of the incandescent lamp given in an 
earlier paragraph. (1) Not all the energy supplied to the lamp is 
transformed into radiation; some is lost by conduction and con- 
vection. (2) Only a small part of that radiated is contained in 
the visible spectrum. Much is emitted in waves too long to affect 
the human eye. (3) That part of the radiant energy which is — 
contained within the visible spectrum is not distributed most ad- 
vantageously. Granting that conduction and convection losses could 
be eliminated, the spectral distribution of the radiant energy is an 
outstanding factor to be reckoned with. 

If we confine our attention to the case of the simplest radiating 
solid, viz., the black body (which see), the only opportunity offered” 
to change the spectral distribution is the variation of the tempera- 
ture of the radiator. It can readily be shown that if all the energy 
supplied to a lamp was radiated in the continuous spectrum of 
black-body radiation corresponding to the temperature of highest 
efficiency, the efficiency of the lamp would be approximately 140 
lumens per watt, or 16 per cent of the highest possible efficiency of 
the most efficient monochromatic light, to which we assigned the 
arbitrary value of 100 per cent efficiency. In other words, this 
most efficient black-body radiator would be 18 times as efficient as 
the tungsten lamp. In passing, it is significant that the tempera- 
ture of the black-body under this condition is that corresponding — 
roughly to the temperature of the sun. In other words, a black 
body at the temperature to produce white light, is at the temperature 
of maximum efficiency for pure temperature radiation. 

In the literature on the general subject of luminous efficiency, 
various phrases indicating different ratios of power and light have 
been invented and used. The confusion that has resulted from the 
use of quite similar terms to signify distinctly different quantities 
suggests in the present treatment the confinement to an explana- 
tion of the important quantities involved, without any extended use 
of the complicated nomenclature. One of the more common ex- 
pressions, the mechanical equivalent of light, has been referred to 


PHYSICAL CHARACTERISTICS OF Luminous SourcEs 39 


already. But even this term, as was: pointed out, is indefinite and 
unsatisfactory as ordinarily used. By the mechanical equivalent 
of light is meant the hght value of radiant energy, where only that 
radiant energy is included which may call forth the sensation of 
light, 1. e., that portion of the radiant energy which lies within 
the limits of the visible spectrum. 

As has been stated in a previous paragraph, the indefiniteness 
in the mechanical equivalent of lght arises from the fact that 
the mechanical equivalent for every wave-length of light, and hence 
for every different composite light, is different. The light value 
for any one wave-length is much more definite, and the determina- 
-tion of the light value for energy of the wave-length, A=0.545 p, 
of maximum sensibility (at high intensities) is of prime importance 
as indicating the upper limit of efficiency theoretically obtainable. 
This quantity, which may be denoted by M, is not known accurately, 
but has an approximate value of 800 lumens per watt, or 0.015 
watt per mean spherical candle. 

The principal relations which excite interest in a study of the 
efficiency of light sources may be stated briefly. Given a definite 
quantity of energy supplied to a lamp: (1) What proportion of 
that energy is transformed into radiation, and what part is dissi- 
pated in other ways, being thus lost so far as its light-producing 
power is concerned? (2) Of that energy transformed into radia- | 
tion, what proportion is contained within the limits of the visible 
spectrum, and is thus productive of light in varying degrees? 
(3) What is the light-giving power of that energy radiated within 
the wave-length limits of the visible spectrum, or, in other words, 
what is the mechanical equivalent of light for that particular lamp? 
Of these three relations the first is quite definite and of consider- 
able importance, whereas the second and, consequently, the third 
are more or less indefinite owing to the ill-defined limits of the 
visible spectrum. If the red end of the spectrum is taken as 0.8 u 
instead of 0.76 » no appreciable difference would be observed in 
the light flux owing to the almost negligible luminosity of energy 
between these limits of wave-length. But the amounts of energy 
ascribed to the visible spectrum in the two cases would be different 
by many per cent for most ordinary light sources. 

The greatest interest, from a practical standpoint, centers not 
in the individual steps of the above analysis, but in the resultant 
ratio of luminous flux available from a lamp in proportion to the 


40 ILLUMINATING ENGINEERING 


power supplied to the lamp. The various steps in the analysis are, 
however, of considerable importance in indicating for any light 
source its most pronounced deficiency. 

The various relations can be represented briefly by the use of 
symbols. Let Q be the power supplied to a lamp, measured in 
watts; R the power radiated, measured in watts; L the power radi- 
ated within the visible spectrum (from A=0.38" to A=0.76 p 
taken arbitrarily), measured in watts; and ¢ the luminous flux 
from the lamp measured in lumens (47 spherical candles). The 


first of the three ratios given above is represented by os , the second 
by ¢ , and the third by § . Unter Q, the power supplied to the 


lamp, would come the power lost by conduction Cg, the power lost 
by convection C,, and the power radiated R, so that Q=Ca+C,+R. 
If the analysis is carried further, as in the illustration afforded by 
the combustion of acetylene, the total power involved in the re- 
action may be represented by Q’, where Q’/=Q+C,, the latter 
symbol indicating the rate at which energy is stored up in the 


resultant molecules of CO, and H,O. Although the ratio a of 
resultant luminous flux to total power involved in the transforma- 
tion would give the ultimate efficiency of the light process, such a 
definition would be comparable with that for an incandescent lamp 
in which not only the heat and other losses in the generation of 
electric power, but even the chemical reactions in the fire-box under 
the boiler * are included in the energy supplied. Such an elaborate 
analysis would take us beyond the logical limits of a discussion of 
the physical characteristics of light sources, and hence will not be 
attempted in these lectures. 

This general discussion of the elements entering to determine 
the efficiency of light sources is intended to prepare the way for 
the more detailed discussion of definite ight sources in the second 
lecture. Under the treatment of each source the data on efficiency 
will be given, in all possible cases. Apart from the mere analysis 
of the efficiency or inefficiency of light sources, our interest should 
carry us further into the study of the causes which underlie the 





* To make the analogy complete it would be necessary to consider the 
energy relations in the generation of acetylene, since this is a manu- 
factured product. | 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES 41 


phenomena exhibited by the lamps. Much valuable knowledge 
is gained from a study of radiation, the laws of radiation and the 
radiating properties of matter. 


3. Quality of Inght 

In the previous lectures in this course, a discussion of color 
of natural objects was given. In this discussion it was assumed 
that the incident light was white light, normally produced, as 
in the case of sunlight. A quite different question, and one of 
distinct importance to the illuminating engineer, is that of the 
quality of the hght furnished by various types of luminous sources.’ 
The quality of the light manifests itself in two ways: (1) in the 
color of the light itself, when the lamp is viewed directly, or in 
the apparent color of white objects when seen illuminated by the 
light; (2) in the apparent colors of various differently colored ob- 
jects when seen illuminated by the hight. Both of these mani- 
festations can be predicted for any light when its spectral composi- 
tion is known. 

Two lights may both appear white and yet have quite different 
spectral compositions. Taking average mid-day sunlight as stand- 
ard for white light, an ordinary solid body, such as carbon, would 
emit a white light if it could be heated to a temperature of 5000° 
or 6000°. The spectrum of such a body would be continuous, and 
approximately the same as that of a theoretical black body (which 
see) at the same temperature. 

On the other hand, a white light can be obtained by the admix- 
ture, in the proper proportions, of red, green and blue light, if 
for these three colors the right wave-lengths are chosen, or by the 
admixture of properly chosen pairs of spectral colors. From a 
mere visual inspection of the luminous source itself, or of a white 
surface illuminated by it, it would be impossible to tell the true 
nature of the white light. But if objects of various colors, when 
viewed under normal daylight, are illuminated successively by the 
light from the two apparently white sources, they would appear 
quite different under the two lghts. Illuminated by the white 
light from the incandescent carbon-at high temperature, the colored 
object would. appear the same as when viewed in daylight. But 
when illuminated by the white light composed of three primary 
colors they would assume new and strange tints. It is not sufficient 
then to adjudge a light good or bad on the basis of its composite 


42 ILLUMINATING ENGINEERING 


appearance. A spectroscopic analysis is necessary to show whether 
the spectrum is continuous or discontinuous, and if discontinuous 
whether the discontinuity consists of a few bright lines scattered 
through the spectrum, as in the case of the mercury arc, or of a 
very large number of bright lines distributed throughout the entire 
spectrum, as in the spectrum of CO, at low pressure. A source 
with a discontinuous spectrum of the latter type is for most prac- 
tical purposes equivalent to a source having a continuous spectrum 
of the same composite quality. In addition to the knowledge of 
the spectral distribution in the hight from two sources of the same 
composite quality, it is of interest to study the composite qualities 
of the various illuminants. These differ greatly among themselves, 
in most cases the ight being distinctly more yellowish than average 
daylight. The composite quality of any light may be expressed in 
terms of the quantities of three primary colors, red, green and blue, 
necessary for a match in color with the light under investigation, 
taking the quantities necessary to produce the white hight of average 
daylight, as red 33 per cent, green 33 per cent, and blue 33 per cent. 

In the detailed discussion of the various artificial illuminants 
in the next lecture, data will be given in all cases where such ob- 
servations have been published, on the quality of the composite light, 
as determined by colorimetric measurements, and also on the dis- 
tribution of energy in the visible spectrum as given by spectro- 
photometric analysis. 


4. Temperature Radiation 


Frequent reference has been made in the previous paragraphs to 
the various elements which enter to determine the ultimate lumi- 
nous efficiency of any light source. As a first criterion for high 
efficiency it was found that the losses of energy by conduction and 
convection should be as small as possible, in order that most of 
the energy suppled the lamp should be transformed into radiation. 
But even though all the energy supplied to a lamp were trans- 
formed into radiant energy, the resultant luminous efficiency might 
range from 0 per cent to 100 per cent, depending upon the distri- 
bution of the energy in the spectrum of the radiating body. The 
study of radiation—the laws of radiation and the radiating prop- 
erties of matter—is therefore of prime interest and importance in 
considering the physical characteristics of luminous sources. 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES 43 


It is necessary to distinguish two kinds of radiation: (1) tem- 
perature radiation, and (2) luminescence. Every body radiates 
energy at least in the form of heat radiation of long wave-lengths. 
If a body, during this process of radiation does not change its 
nature, it would continue to radiate in the same way if its tem- 
perature were maintained constant through the addition of heat. 
Such radiation is ordinarily known as temperature radiation. On 
the other hand, if a body undergoes change during the process of 
radiation, it would not in general continue to radiate in the same 
way, even though its temperature were maintained constant through 
the addition of heat. Such a process of radiation is known as 
luminescence. 

Considering first temperature radiation, which plays an important 
role in determining the luminous efficiency of practically all ordi- 
nary illuminants, and which determines entirely the efficiency of 
electric incandescent lamps, it is necessary to introduce the idea 
of the theoretical black body,’ or complete radiator, as it is some- 
times called. All natural bodies show individual peculiarities in 
their radiation, and it is therefore desirable to refer back to some 
simple standard radiator. 

We ordinarily call an object “ black” which seemingly reflects 
little or none of the light incident on it. Exact measurement would 
show that each such object actually does reflect some light, and, 
moreover, in general that it reflects relatively more energy of some 
wave-lengths than of others. In other words, it reflects selectively. 

A theoretical black body absorbs all the energy of every wave- 
length throughout the entire extent of the whole spectrum, i. e., 
it reflects none of the energy incident on it. 

According to a law first formulated by Kirchhoff, and known by 
his name, the quantity of energy radiated per second by any body 
at any temperature is proportional to the absorptive power of the 
body at that temperature. Thus, given two bodies, A and B, such 
that at some definite temperature the coefficient of absorption for 
body A for energy of some definite wave-length is double the cor- 
responding coefficient for B; then at the same temperature body A 
would radiate per unit area per second twice the amount of energy 
of the given wave-length radiated by B. Since the black body 
absorbs all the energy incident on it, it will conversely, at any 
temperature, radiate more energy of every wave-length per second 
than any natural body. 


4b ILLUMINATING ENGINEERING 


The relatively simple properties of the theoretical black body 
have inspired several attempts at theoretical deductions of the laws 
of black-body radiation. Moreover, in recent years the theoretical 
black body has been quite closely approximated by the use of a 
hollow cylinder insulated as far as possible from the surrounding 
air, and having a small aperture at one end through which the 
radiation from the interior walls of the cylinder escapes. Such a 
body, heated uniformly by an electric current, emits radiation ap- 
proximating quite closely, both in quality and in quantity, that 
emitted by a true black body at the same temperature. Hxperi- 
ments carried out with radiators of this type have corroborated in 
a general way the black-body radiation laws deduced theoretically. 
There still remains, however, considerable uncertainty as to the 
exact values of the constants entering in the mathematical expres- 
sions of the laws, and in the case of the law of spectral distribution 
of energy at any given temperature the exact form of the law is 
not yet satisfactorily established. As a discussion of the laws of 
black-body radiation is included in another lecture of this course, 
they will not be given here. Constant reference will be made, how- 
ever, to the properties of black-body radiation as a convenient 
standard with which to compare the radiation from natural bodies. 
Even though the radiation from a natural body may be due entirely 
to the temperature of the body, the quantity and quality of the 
energy radiated by material bodies at the same temperature depend 
on -the nature of the bodies themselves. Only in the case of an 
absolutely black body is the radiation simply a function of the 
temperature. . 7 

Without introducing mathematical analysis it is instructive to 
consider briefly the changes produced in the quantity and quality 
of energy emitted by a black body corresponding to change in tem- 
perature. Such a consideration will conduce to a proper apprecia- 
tion of the importance of attaining the highest possible tempera- 
tures if high luminous efficiency is to be secured. It has already 
been stated that the highest possible efficiency obtainable from a 
black body is but 16 per cent of the highest possible efficiency of 
monochromatic hght. Moreover, even this is only obtainable at the 
extremely high temperature of 5000° or 6000°, a temperature far 
in excess of any that has as yet been realized in any lamp. It is 
of interest, therefore, to investigate the relation between the lumi- 
nous efficiency and the temperature of a black body at various tem- 


PHYSICAL CHARACTERISTICS OF LumINOoUS SouRCES 45 


peratures. This relation depends on the relative amount of energy 
radiated in the visible spectrum compared with the total radiation, 
and on the way in which the energy in the visible spectrum is 
distributed. | 


























ia 
CN ee 
el TT | Naren re 
| 2): SoS Main 
| Tol f BAN 
| THT YY AAR [| a 
Fo. o0\ (eee 
eax errr 
tb SeWal GR RENEE |_| 
tif | A AN AN 























SSS iz 
St _ = 





Fig. 3 me erey Curves for a Black Body at Various Temperatures 
(Absolute). 


Note—To obtain the proper relative intensities of radiation, the ordinates of 
curves b, c, d, e, fand g must be divided by 8, 6, 15, 50, 100 and 1000 respectively. 


At very low temperatures the energy radiated per second in the 
visible spectrum is too small to affect the eye. As the temperature 
is increased the total energy radiated increases rapidly and the 
rate of increase is most rapid for the shorter wave-lengths such 
as affect the eye. Thus, at temperatures in the neighborhood of 


46 ILLUMINATING ENGINEERING 


1900° and 2100° Abs., when the temperature is increased 1 per 
cent the total energy radiated is increased 4 per cent, whereas the 
energy radiated in the visible spectrum is increased about 10 per 
cent or 15 per cent. Consequently, as a result of 1 per cent rise 
in temperature there is an increase of 8 per cent or 10 per cent 
in efficiency, i. e., in lumens per watt radiated. 


50 











PPCECLELETy SOO 
BRERA SRERRREABREREREMe.. 2. 
oe bos ee eee 
































ue 
Jo) for te 


ech dia 
SR/RRERERES CH 
LA le ea ee 
ART Sane e eS 





























| 
ae 
eee 4H Ra 


SAeee 
bbe TACT AE oe eee 
pt ert Lol Te | heehee ose ea tae 
He on 
ol BEER aeP .SRRRNAR ARNE Yee 
—— 


1000° 2000° 4000° 
abit Temperature. 
























Fie. 4.—Values of “| rs and é for a Black Body at Various Tem- 


nanan 


In Figure 3 are plotted curves showing the relation between 
' energy radiated and wave-length for various temperatures ranging 
from 1000° to 8000° Abs. Below 1000° Abs. the energy in the 
visible spectrum is practically negligible. As the temperature is 
increased, relatively more and more of the energy is radiated in 
the visible spectrum, the position of the maximum emission shifting 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES AT 


constantly toward shorter wave-lengths. At a temperature of 
about 6000° the maximum lies in the visible spectrum, and ap- 
proximately at that wave-length in the visible spectrum corre- 
sponding to the maximum sensibility of the eye. 

As the temperature is increased beyond 6000° the maximum is 
displaced still further toward shorter wave-lengths, and the pro- 
portion of energy in the visible spectrum begins to decrease. There 
is, therefore, for a black body, a temperature of maximum efficiency 
beyond which the efficiency falls off again. Inasmuch, however, 
as all illuminants in present use which depend on temperature for 
their efficiency are operating at temperatures very much below that 
of maximum efficiency, any improvement which would make pos- 
sible the use of higher temperatures would conduce to higher 
efficiency. 

If the limits of the visible spectrum are taken as A=0.76 » on 


the red side,-and A=0.38 p» at the violet end, the ratiog of energy 


radiated in the visible spectrum to total energy radiated by a black 
body at various temperatures can readily be computed. The results 
of such a computation are plotted in curve “a,” Figure 4. The 
abscissae are temperatures, and the ordinates are the corresponding 


values of the ratio Z . It is thus seen that for the arbitrarily 


chosen limits of wave-length of the visible spectrum given above, 
the greatest proportion of energy is radiated in the visible spectrum 
when the temperature of the black body is a little over 6000° Abs. 


' Even at this temperature, however, the absolute value of 7 is only 


50 per cent. 
It cannot be assumed a priori, however, that the highest luminous 


efficiency corresponds to the greatest value of , for the distribu- 


tion of the energy in the visible spectrum as compared with the 
sensibility curve of the eye determines to a large extent the lumens 


per watt radiated, i. e., the ratio . In other words, Fis the 
resultant of the product of the two ratios ¢ x aa , and the maxi- 


mum of the product will agree with the maximum of one of the 


terms only in case the two ratios eiendae have their maxima at 


L 
the same temperature. 


48 | ILLUMINATING ENGINEERING 


As has already been explained, ois the ratio of the luminous 
flux measured photometrically to the corresponding energy flux 
(between the wave-lengths A=0.38 » and A=0.76 »), measured in 
watts, and depends for its value on the distribution of the energy 
in the visible spectrum. If all the energy were concentrated at 
the wave-length of maximum sensibility (A=0.545 w) the values 


of the ratio oi would be M=800 lumens per watt. Calling this 
| $ 


maximum value unity, the values of ae corresponding to a black 


body at various temperatures are given in curve “b,” Figure 4. 
From an examination of this curve it is seen that for black-body 


db 


radiation the ratio = reaches a maximum at a temperature of 


L 


about 5000° Abs., or not greatly different from that at which = 


has its maximum.’ In other words, it so happens that for black- 
body radiation the temperature at which the largest proportion of 
the radiant energy lies in the visible spectrum, is also the tempera- 
ture at which the distribution of energy in the visible spectrum is 
most favorable for the production of light. Since this tempera- 
ture, 5000°-6000°, is approximately that of the sun and corre- 
sponds to white light, the conclusion follows that owing to the 
peculiar sensibility curve of the human eye, possibly due to inherited 
ancestral adaptation, the sun is at the temperature of ara pos- 
sible luminous efficiency for a black-bedy radiator. 

From an inspection of curve “b” in Figure 4, it is seen that 
even at the temperature of maximum efficiency the lumens produced 
by one watt radiated in the visible spectrum is only 33 per cent 
of the lumens that would be produced if all of the energy were 
concentrated at the wave-length (A=0.545 ») of maximum sensi- 
bility. Multiplying the ordinates of curves “a” and “b” in 


Figure 4, there is obtained the value of e , ( ‘ ay Pie 4) , for 





Ki”) es eee 

a black body at various temperatures. ‘This new curve thus ob- 
tained is plotted in curve “c,” Figure 4. It is seen that the 
maximum efficiency obtainable from a black body (at the tempera- 
ture of the sun) is only 16 per cent of the highest possible efficiency 
of monochromatic radiation. at 

Inasmuch as temperature plays so important a part in deter- 
mining luminous efficiency, it is of interest to consider briefly the 


PHYSICAL CHARACTERISTICS OF LUMINOUS SouURCES 49 


possibilities in the employment of high temperatures as fixed by 
the melting points of available substances. The melting points of 
elementary substances seem to follow a periodic function of the 
atomic weights.. Thus, carbon, tungsten, tantalum and thorium 
all lie at periodic maxima of melting-point temperatures. _ The 
element which has the highest melting point is carbon, which has 
figured prominently in artificial illumination from the earliest days, 
as in flames and in the carbon incandescent and are lamps. But 
the employment of carbon suggests another consideration which 
enters. in the choice of a filament for use in an incandescent lamp. 
High melting point does not avail much if the vapor tension of 
the material is so high that the filament evaporates rapidly at 
moderately low temperatures. This element conditions the tempera- 
ture practicable in an incandescent lamp, and although “ flashing ” 
and “ metallizing”’ have tended toward reducing the vapor tension, . 
and thus made possible higher working temperatures, the tempera- 
ture that can be employed is still low compared with the melting 
point of carbon. 

The oxides and silicates, particularly of the rare earth metals, 
form a group of highly refractory substances which have been em- 
ployed in lamps (e. g., the incandescent mantle and the Nernst 
elower) partly because of their refractoriness and partly on account 
of the peculiar nature of their emission spectra. 

We have.seen that there is an upper limit to the possible effi- 
ciency of a black body, and that this upper limit is relatively low. 
Moreover, at present there are no means known for even approach- 
ing the temperature at which the highest efficiency is secured. What 
other possibilities are there, then, of obtaining high-efficiency lamps? 
Excluding luminescence, which will be discussed later, and con- 
fining our attention to-temperature radiation, the answer to this 
question, if, there is an answer, must be found in the phenomenon 
of selective radiation." 

So far as is known, no body in nature radiates ssieisiedlly as a 
black. body. No material body absorbs all of the energy of any 
wave-length incident on it; hence a black body,, from its very 
definition, must, at.a given temperature, emit more energy of every 
wave-length than any other body at the same temperature. | Con- 
sequently, a material body may differ from a black body in that it 
emits per unit area at a given temperature a smaller quantity, as, 
for. example, one-half or one-third of the energy of every wave- 
length of that emitted by the black body at the same temperature. 


50 ILLUMINATING ENGINEERING 


The energy curve of such a body would be identical with that of 
a black body except that its ordinates would be reduced proportion- 
ally throughout the entire spectrum. Such a body is sometimes 
known as a gray body. It can be realized experimentally by inter- 
posing between an experimental black body and the screen on which 
the radiation falls a rotating sectored disk. If the total aperture 
of the disk were 180° it would*reduce by one-half the energy of 
every wave-length received from the black body. If the aperture 
were 90° but one-fourth of the energy of the black body would be 
received. In both cases the radiation received on the screen, i. e., 
the radiation emitted by the black body and sectored disk, con- 
sidered as a unit, would be that of a gray body, that is, the same 
in quality but less in quantity than that of a black body at the 
same temperature. It is evident that there can be an infinite 
. number of gray bodies corresponding to a black body at any given 
temperature. The various gray bodies would differ from one 
another and from the black body in total emissivity. 

The importance of the distinction between the black body and 
the gray body arises from the fact that not infrequently the emis- 
sivity of a substance is cited in partial explanation of high efficiency. 
It is true that some substances which have low emissivities exhibit 
also the property of selectwity, which will be discussed presently, 
but it should be emphasized that mere grayness or difference in 
total emissivity has no direct influence on the efficiency of the 
radiation from a substance possessing this property. A gray body 
is no more or less efficient than a black body at the same tempera- 
ture. The quality of the radiation is the same for both. The 
ratios z and ¢ would be identical for both. In the case of two 
filaments of the same size, one black and one gray, it would be 
necessary,.in order to bring both to the same temperature, to 
supply more energy, say two or three times as much energy to the 
black filament as to the gray one. But the luminous flux obtained 
from the black filament would be twice or three times that emitted 
by the gray filament. It is a question of the difference between a 
32c.p. and a 16c.p. lamp at the same watts per candle. 

An indirect practical advantage in the use of substances having 
low emissivities in the manufacture of lamps is that, owing to the 
lower emissivity, filaments of larger size for any given candle- 
power may be used, thus making possible stouter and stronger 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES 51 


lamps. Another advantage is that the filament or mantle of lower 
emissivity would have a lower intrinsic brightness than a black 
filament or mantle at the same temperature. 

But a material body can differ from a black body in its radiating 
properties in another way. Not only may the quantity of energy 
emitted be different from that of a black body at the same tempera- 
ture, but the quality may also be different. Thus, if a body emitted 
one-fourth as much red, and one-third as much green, and one-half 
as much blue as a black body at the same temperature, it would 
not correspond to a gray body, but would radiate in a way that is 
known as selective; that is, it would radiate relatively more energy 
at one wave-length than at another compared with a black body 
at the same temperature. This type of selectivity is to be dis- 
tinguished from that kind of selective radiation exhibited in the 
bright line spectra of luminous gases. 

All substances which have been investigated show deviations 
from the ideal black body in respect both to the quantity and the 
quality of the radiation. It is therefore a matter of great interest 
and importance in the consideration of the physics of light produc- 
tion to study the radiating properties of matter, and, if possible, 
to correlate these with the other properties of elementary substances. 
Although much investigation has been directed toward the solution 
of these problems, the results obtained thus far are relatively 
meager. Universal agreement on the laws of black-body radiation, 
the simplest case, has not yet been reached, and the investigation 
of the peculiarities of the radiation from matter is but just begun. 

It is beyond the scope of this lecture to discuss in detail the 
methods that have been employed, and the results that have been 
obtained in the investigation of the radiating properties of matter. 
Closely linked with the radiating properties of a substance are its 
reflecting properties, as formulated in Kirchhoff’s law, and much 
valuable information regarding the radiating properties has been 
obtained by this indirect method, supplementing the results of 
direct investigation. The fundamental difficulty in arriving at 
definite conclusions regarding the radiating properties of matter 
at high temperatures is that of measuring the temperature. Meth- 
ods have been employed, however, which indicate quite certainly 
in a qualitative way the relative selectivity of the radiation from 
various substances, and which, with the aid of some simple and 
probable assumptions, give an idea of the magnitude of the dif- 
ferences. 


52 ; ILLUMINATING ' ENGINEERING 


It is perhaps worth while to suggest at this time the practical 
importance of selective radiation. We have seen that differences 
in emissivity have only an indirect effect in the production of 
efficient lamps. A gray body is no more or less efficient than a 
black body at the same temperature. Selectivity,'on the other hand, 
plays a direct, and possibly in certain cases an important, rdle in 
determining the efficiency of a lamp. Given a number of sub- 
stances which can be operated at the same temperature, that sub- 
stance would be most efficient as a luminous radiator, other things 
being equal, which radiated the largest percentage of energy in the 
visible spectrum and with the energy in the visible spectrum so 
distributed as to produce the greatest light effect. If a substance 
could be found which radiated all the energy in the visible spec- 
trum, and so distributed as to produce white light, an ideal lamp 
would result. No substance has been’found in which these condi- 
tions are approached, but investigation has shown that for some 
substances, e. g., the filament of the osmium lamp (which see), 
‘selectivity is of quite appreciable significance in determining 
efficiency. , | 

From the standpoint’ of luminous efficiency that type of selec- 
tivity is of interest in which the emission in the visible spectrum 
is exaggerated compared with that radiated in other wave-lengths. 
On the other hand, substances exist which would be much less 
efficient as luminous radiators than a black body at the same tem- 
perature. For example, ordinary glass, if it could withstand the 
temperature of carbon filaments, would be much less efficient than 
a carbon lamp, assuming that the radiating properties of glass 
would not undergo serious change at higher temperatures. At 
ordinary temperatures glass absorbs very little energy in the visible 
spectrum compared with that absorbed in the deep infra-red. Con- 
versely, glass emits relatively much less energy in the visible spec- 
trum than a black body: at the same temperature. Such a sub- 
stance is ill-fitted to serve as a luminous radiator. 

At the present time there are not sufficient data on the radiating 
properties of substances to justify any extensive classification.. As 
a rule metals show relatively low-reflecting powers in the visible 
spectrum and uniformly high-reflecting powers in the infra-red. 
Conversely, such metals would show a relatively higher emission 
in the visible as compared with the infra-red spectrum, and would 
hence be more efficient luminous radiators than a black body at 
the same temperature. 


PHYSICAL CITARACTERISTICS OF LuMINOUS SoURCES 53 


In studying the incandescent mantle and the Nernst glower in 
the next lecture’ the peculiar form of the spectra of the oxides 
composing these radiators will be discussed. It has been proposed 
by some investigators that the radiation from these substances is 
not to be ascribed entirely to the temperature, but is due - in part 
to luminescence. 


5. Luminescence 

If a body during the process of radiation undergoes a change in 
nature, it would not in general continue to radiate in the same 
way even though its temperature were maintained constant through 
the addition of heat. Such a process of radiation has been defined 
as luminescence. The cause of the radiation in this case is con- 
sidered to lie not in the temperature of the system, but in some 
other source of energy. It.is a simple matter to adduce illustra- 
tions of apparently typical at toe and of typical tempera- 
ture radiation, but in many cases the distinction is difficult if not 
impossible. Even in apparently typical cases of luminescence it is 
a question as to whether the ultimate cause of the radiation may 
not be temperature—not the average temperature of the system, but 
the high localized temperature in isolated’ portions of the system. 

The definition of luminescence that has been given is taken 
from Drude, and differs slightly from the original definition of 
Wiedemann, who first introduced the term. In the light of more 
recent experiments and more modern theory it is questionable 
whether either definition is illuminating or helpful to a better 
understanding of the phenomena. In the process of light produc- 
tion by the passage of an electric current through the filament of 
incandescent lamps we commonly say that the electric energy is 
transformed into heat, and that the filament is heated to such a 
temperature that it becomes incandescent. On the other hand, in 
the case of the luminous vapor in the are discharge we frequently 
say that the vapor radiates by luminescence, and that there is a 
direct transformation of electric energy into radiation without the 
intermediate form of heat energy. And yet in this case, as in 
the other, there is some intermediate form of energy, viz., the 
kinetic energy of the corpuscles or ions which are mete by 
the electric force. 

Whether or not there is any fyi ae aniegnie: between Paoade 
ture radiation and luminescence in true physical significance there 


54 ILLUMINATING ENGINEERING 


is unquestionably a marked difference in the apparent phenomena 
exhibited in the two cases. In the discussion of the various light 
sources in the second lecture following general custom the term 
‘luminescence willbe used to describe that type of radiation which 
it is claimed by some has never been produced by heating the sys- 
tem * as a whole, but throughout the term will be used with re- 
serve as to its exact significance. 

Drude™ classifies under the general term luminescence the fol- 
lowing phenomena: (1) Chemi-luminescence, as in the glow from 
slowly oxidizing phosphorus; (2) photo-luminescence, ordinarily 
known as phosphorescence, which is the after-glow resulting from 
previous radiant excitation; (3) electro-lwminescence, as in the glow 
from Geissler tubes. Under electro-luminescence would also come 
the luminescence from the vapor in the are discharge. 


Lecture II 
1. Introduction 


In the preceding lecture a general discussion of the various ele- 
ments which enter in a study of the physical characteristics of 
luminous sources was undertaken. In the present lecture the 
different types of illuminants will be discussed in regard to the 
various elements presented in the first lecture, so far as the peculiar 
natures of the different illuminants and the available data will 
permit. It is of interest to notice how investigation has been pur- 
sued along different lines for the different sources, making a well- 
balanced analysis difficult, if not at times impossible. It has been 
the aim in this lecture to consider each illuminant in the same 
general way, and at the same time to emphasize the peculiar and 
interesting physical properties of each. 

Although in general the various physical properties of each il- 
luminant are presented in the discussion of that particular illumi- 
nant, exceptions have been made in the matter of spectral distri- 
bution and quality of light, since it seemed that this information 
would be of more value when collected in a comparable manner. 
These questions therefore are discussed in separate sections at the 
end of the lecture. 


* In a recent note in the Comptes Rendus (Vol. 130, No. 26, p. 1747; 
June 27, 1910) Bauer reports an experiment in which the characteristic 
bright line spectrum of sodium vapor was in his opinion obtained by 
heat. 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES 55 


2: The Physics of the Electric Incandescent Lamp” 


In many ways the electric incandescent lamp is the simplest lamp 
that could be constructed, speaking from a physical standpoint. 
This is particularly true of the older form of untreated carbon 
lamp, since untreated carbon approaches quite close to the theoret- 
ical black body in its radiating properties. Untreated carbon has 
a high emissivity, and exhibits very slight indication of any selectiv- 
ity in its radiation. 

For the electric incandescent lamp in general it may be said that 
all the energy supplied to the lamp is transformed into heat, prac- 
tically all of the energy into heat in the filament itself, since the 
resistance of the leading-in wires is in all practical cases quite small. 
Of the energy transformed into heat in the filament nearly all is 
radiated. As we shall see later the losses by thermal conduction 
along the leading-in and anchoring wires, and the conduction and 
convection by the enclosed gas at very low pressure, are very small 
in commercial lamps. Finally, the radiation is pure temperature 
radiation, there being no luminescence, and, in the case of the un- 
treated carbon filament, the efficiency is due almost entirely to the 
temperature, there being no appreciable selectivity. The metal- 
filament lamps show marked evidence of selective emission, which 
in part determines their efficiency, but with this exception are quite 
similar to the untreated carbon-filament lamp in all respects. 

‘In the introduction to the first lecture the incandescent lamp was 
cited to illustrate the principles involved in the physics of a lamp. 
In this illustration an experiment was described showing the tre- 
mendous losses that would result from conduction and convection 
by gas in the bulb. Under the conditions of the experiment which 
was performed upon a platinum filament (0.1 mm. diameter, 15 cm. 
long) in a pear-shaped bulb (13cm. long and 8cm. maximum 
diameter), the power required to bring the filament to a tempera- 
ture of about 1700° Abs. when the bulb was filled with air at at- 
mospheric pressure was found to be about five times that required 
_ to bring the filament to the same temperature in a vacuum. 

The curves in Figure 5 show the relation between the required 
power and the pressure inside the bulb for the same platinum lamp 
and the same temperature. It is seen that the required power 
changes most rapidly at moderately low pressures, and that the 
loss due to the gas is still quite appreciable at pressures as low as 
0.05mm. mercury. It should be emphasized that the numerical 

3 ; 


56 ILLUMINATING ENGINEERING 


results found depend largely on the conditions of the experiment. 
A difference in size of bulb or composition of filament, or tempera- 
ture of filament or composition of gas, would probably affect the 
numerical results to a marked extent. The curves of Figure 5 are 
given to show the general nature of the phenomenon. 

As stated in a previous paragraph, this loss by conduction and 
convection of the enclosed gas in a commercial lamp is negligibly 
small. The vacuum that is secured in a good lamp is probably of 

















300 
Pressure 


400 S00 
in’ mm of Mercury. 


























As a eat oH = cles 
HH Mi penton 
Rane ZEEE 
Z 
Pree SEER EE EEE 
a Wis || 















BSS Beas 
BRGWREoND Cores HH RR MBEr Baio 
| | A pode [et Tb eo ae ae Hn p 
EPEC EC EPS et eee ae ee dita Sele 
PT TET et eT Teel Des TT Teeth STS etal 3] Tete eae ere 
VRRP ARRAS 
PT Ie OA Ee See ee 
Rea SaNe 4h Ree ace. || 
ptt LA 


RABE? 

BRERY ABE aise bie i 

BaP 4B PRR PPE 
pw op ep FPRASeS 
ts sl dl RRRBRRERRRRER RGR. 
SAS tte ee 
400 RERERRRERERRERRRRERE 
PLT PP PPE Eph EE ee ee 


practice iio mm, st ender Phir b) 



























ore 


° 


Fig. 5.—Watts Required at Varying Air Pressures to Maintain a 
Platinum Filament at a Constant Temperature (that of a Color Match 
with a Black Body at 1690° Abs.). 


the order of magnitude of 0.001 mm. mercury, which is smaller 
than any vacuum measured in the described experiment. 

The losses, on the other hand, by conduction away of heat at the 
leading-in and anchoring wires, though relatively small, are not 
negligible. Measurements of the total energy radiated, compared 
with that supplied, have led several investigators to the conclusion 
that as much as 20 per cent or 30 per cent of the supplied energy 
is lost in this way. Recent measurements on the temperature gradi- 


* 


PHYSICAL CHARACTERISTICS OF LUMINOUS SouURCES 57 


ent of filaments near the leading-in and anchoring wires have led 
to the conclusion that the loss by conduction in an ordinary 1.25 
w. p. c., 110-volt, 25-watt Mazda lamp is not more than 5 per cent. 
For a 2w.p.c., 110-volt, 40-watt tantalum lamp the loss is less 
than 7 per cent. The larger loss for the tantalum lamp is due 
probably to the relatively larger number of anchor wires. 

The differences in efficiency among the various incandescent lamps 
are to be ascribed therefore to the differences in the quality of the 
radiation.” Since there is no luminescence the quality of the radia- 
tion results from (1) the temperature at which the filament oper- 
ates, and (2) the selectivity of the radiation corresponding to the 
filament material. Unfortunately, it is impossible to separate these 
two elements entirely. The measurement of temperature involves 
in general certain assumptions regarding the nature of the radia- 
tion, and the measurement of selectivity depends upon temperature 
relations. Methods have been devised, however, which give quali- 
tative indication of the relative selectivity of the various filaments, 
and which on the basis of probable assumptions indicate the lower 
quantitative limits to the selectivity. 

It is beyond the scope of these lectures to enter upon a discussion 
of the methods employed or of the detailed results obtained. The 
principal conclusions regarding the selectivity of lamp filaments 
which have been reached up to the present time may be stated as 
follows: If the various metal-filament lamps were operated at the 
same temperature as an ordinary untreated carbon filament at 4 
watts per candle, then it is probable that owing to selectwity the 
tantalum lamp would have a higher efficiency than that of the 
carbon by more than 10 per cent or 12 per cent, the tungsten lamp 
would have a higher efficiency by more than 25 per cent or 30 per 
cent, and the osmium lamp would have a higher efficiency by more 
than 40 per cent. Of any of the metals, platinum, tantalum, tung- 
sten or osmium, the last seems to differ most widely in the quality of 
its radiation from the black body. Both tantalum and tungsten 
give evidence of greater selectivity than platinum. Platinum devi- 
ates far from the black body in its emissivity, but, as explained 
above, this has only an indirect influence on its fitness for use as 
a lamp filament. | 

The efficiency of all the metal lamps is therefore probably due in 
part to selectivity. Only in the case of the osmium filament, how- 
ever, is the selectivity so great that its high efficiency (1.5 w. p.c.), 


58 ILLUMINATING ENGINEERING 


as compared with a 3.1 w.p.c. carbon lamp, is almost entirely ex- 
plained on the basis of the selectivity of its radiation. If the 
conclusions cited above are correct the temperature * of the osmium 
lamp is probably not very different from that of a flashed-carbon 
lamp at 3.1 watts per candle. 

Inasmuch ‘as practically all of the energy supplied to incan- 


descent lamps is radiated, the ratio 4 is practically unity (more 


accurately about 0.95) for the ordinary commercial lamps. Hence, 


? 


—; may be taken as + , and is the ordinary luminous efficiency — 


R 


for the various types of lamps at normal burning expressed in 


lumens per applied watt. The values of = may be taken roughly 


as ranging between 0.02-0.03 for a 4-watt carbon lamp, to 0.08-0.10 


for a 1.25-watt tungsten lamp. Of course, the ratios 7 and: iP 


Q 
can be modified at will by operating the lamps at different tem- 


peratures, i. e., at different voltages, but these cases do not have 
any interest to us at present except in so far as the relations be- 
tween voltage, resistance and candle-power are of great importance 
in practical operation. On account of their importance in opera- 
tion these characteristics will probably be considered in another 
lecture. Suffice it to say here that the temperature coefficients of 
resistance of the various filaments differ widely among themselves 
both in sign and amount, ordinary carbon, for example, having a 
negative coefficient at normal temperatures of operation, whereas 
tungsten and the other metals have relatively large positive co- 
efficients. Moreover, owing to the peculiar radiating properties of 
the various filaments, there are marked differences in the changes 
in candle-power corresponding to a given change in total energy 
supplied. 


8. The Physics of the Arc Lamp 


The various types of are lamps have been subject to much in- © 
vestigation, both as to their physical and operating characteristics. 
Many of the physical characteristics of the are are so intimately 


* Owing to the great discrepancies in the published values for the 
true temperatures of the different incandescent lamps, no attempt is 
made to give in this lecture a table of most probable values. Refer- 
ences to original publications on the subject are given in the bib- 
liography.™ 


PHysicAL CHARACTERISTICS OF LuMINOUS SOURCES 59 


connected with its operation that they will undoubtedly receive 
treatment in another section of this series of lectures. I shall there- 
fore confine myself to a brief statement of some of the more purely 
physical characteristics of the arc. 

The arc may be defined for our purposes as a portion of the 
circuit consisting of a pair of electrodes of solid or liquid material, 
electrically connected by a body of vapor which results from the 
volatilization of material from one or both of the electrodes. The 
term “arc” is sometimes used in a more restricted sense to apply 
to the bridge of vapor alone, and also in a still different sense to 
the process of the discharge or flow of current between the elec- 
trodes, rather than to the part of the circuit where it occurs. 

If we connect a volt-meter to the terminals of an are as used in 
lighting, we find that the volt-meter indicates a difference of poten- 
tial of 40 volts or more, depending upon the type of lamp used. 
The difference of potential is determined by the current,” the dis- 
tance between the electrodes, the materials of the electrodes and the 
pressure of the surrounding atmosphere. The fall of potential 
across the arc is not due to mere ohmic resistance, as in a wire 
carrying a current. Under certain special conditions an are may 
be obtained in which the voltage increases with the current, but 
such is not a true are as used in lighting. This kind of discharge 
is sometimes called a “ glimm-strom.” A distinct characteristic of 
a true are is that as the current increases the voltage decreases. 
For this reason it is essential that arc lamps operated on constant 
potential supply circuits should be provided with series-ballast re- 
sistances to prevent the are from short-circuiting.” 

In the case of an ordinary incandescent lamp the fall in potential 
is practically distributed uniformly along the entire length of the 
filament, so that if we should measure the voltage drop along each 
centimeter of the filament we would find it to be the same through- 
out, and equal to that fraction of the total applied voltage which 
one centimeter bears to the total length of the filament. Such is 
not the case with the arc. The fall in potential takes place in three 
distinct steps: (1) at the anode, or where the current passes from 
the positive electrode to the gas; (2) along the vaporous path; and 
(3) at the cathode, or where the current passes from the gas or 
vapor to the negative electrode. The proportion of the total applied 
voltage which is taken up at each of these three places depends 
on the nature of the are. In the ordinary short carbon are™ the 


60 ILLUMINATING ENGINEERING 


greater part of the fall of potential is at the anode, whereas in 
flaming arcs most of the electrical energy is transformed in the 
conducting vapor. 

Corresponding to the three regions where the fall of potential 
takes place, with the corresponding transformation of electric en- 
ergy, are the three distinct regions of luminous radiation, viz., the 
anode, the cathode and the vapor. And just as the distribution of 
the potential drop depends on the nature of the are, so the distri- 
bution of the radiant energy varies greatly. In the case of the short, 
open, direct-current, carbon arc the gas or vapor contributes but a 
very small part” (several per cent) of the total luminous flux. 
The anode and cathode both are raised to a high temperature and 
consequently radiate energy, but the temperature of the anode is 
much higher than that of the cathode, and is to be considered 
practically as the light source in the open carbon are. In the en- 
closed arc the luminous vapor plays a larger part in determining 
the luminous efficiency, but the principal source of hght in the 
direct-current arc is again the anode. In alternating-current arcs 
the two electrodes play equal parts in producing the luminous flux, 
but as the two terminals are alternately positive and negative, and 
as relatively little heat is produced at the negative terminal, the 
average temperature of the carbon electrodes of an alternating-cur- 
rent arc is lower than the temperature of the anode in a direct- 
current arc, and hence the luminous efficiency of the former is less 
than that of the latter. : 

In direct-current and alternating-current open and enclosed car- 
bon ares the electrodes supply the larger part of the luminous flux, 
which therefore may be said to be due to pure temperature radia- 
tion. In the case of the luminous and flaming arcs the principal 
source of the luminous flux is the luminous vapor, the electrodes 
adding little to the luminous efficiency. The difference between 
these two types of arcs, luminous and flaming, is significant in its 
bearing on the physics of light production in these ares. The es- 
sential difference between the two arcs consists in the two distinct 
processes by which the light-giving vapors find their way into the 
are, where they perform simultaneously the two functions of con- 
ducting the current and radiating the light—two functions that are 
doubtless intimately connected. (1) A carbon are may be used as 
a basis, the anode being impregnated with salts or pierced with a 
longitudinal hole through which a metal wire is threaded. ‘This 


PHYSICAL CHARACTERISTICS OF LumINoUs SouRCES 61 


gives the flaming arc. The anode is used because in the case of the 
carbon arc it is the hotter. The vapor is the result of the evapora- 
tion of the salts or metal, and takes part in the conduction of the 
current through the arc. The anode burns away rapidly as a result 
of the high temperature and consequent evaporation. (2) The 
vapor comes from the cathode. Such an arc is called a luminous arc. 
In it the anode may be entirely free from burning or melting away, 
being quite cool. There is necessarily a consumption of the cathode, 
but it may be rather slow. 

The importance of the distinction between these two types of 
arcs enters in the consideration of the possible explanation of the 
high, luminous efficiency of the radiating vapor. Is this efficiency 
to be ascribed to pronounced selective temperature radiation at a 
high temperature or to luminescence? This is a much-mooted 
question, though the consensus of opinion at the present time seems 
to be that the efficiency is to be ascribed to luminescence rather than 
to pure temperature radiation. 

The probability of this theory may be seen from the phenomena 
of the luminous are. In the case of the flaming arc the metallic 
vapors get into the are through evaporation at the anode, indicating 
at least as high and possibly a higher temperature than that of the 
anode. According to Violle* the temperature of the carbon flame 
is higher than that of the anode. With such temperatures it is not 
unthinkable that with vapors showing selective emission in favor 
of the visible region of the spectrum the luminosity might be due 
to selective temperature radiation rather than to luminescence. In 
the case of the luminous arc, however, the conditions are quite dif- 
ferent. The anode may be entirely cold, the vapor being carried 
into the are stream by the process of conduction. It is probable 
that the cathode is always fairly hot, but the evaporation of the 
cathode is unquestionably below that of the anode in the flaming 
are. In the case of the luminous arc, therefore, there are perhaps 
greater difficulties in explaining the luminosity by temperature ra- 
diation, and, on the other hand, more cogent reasons for accepting 
the theory of luminescence. 

Unfortunately, no data are available on the actual temperatures 
of the luminous gas in luminous and flaming arcs. On the other 
hand, numerous measurements of the temperature of the anode 
crater of a direct-current carbon arc have been made, and some little 
data on the temperature of the cathode and vapor have been pub- 


62 ILLUMINATING ENGINEERING 


lished.” The most probable temperature deduced from these obser- 
vations is about 3800°-4000° Abs. (Centigrade+273°). The tem- 
perature of the cathode has been found by Rosetti to be 3300° 
Abs., but as the same observer determined the temperature of the 
positive carbon to be about 4150° Abs. it is probable that his value 
for the cathode is 200° or 300° too high. For the vapor itself the 
value 5000° Abs. has been given. In the alternating-current arc 
the two electrodes have the same temperature, intermediate between 
the temperatures of the anode and cathode of the direct-current are. 

The arc lamp, like many other practical sources, is subject to 
losses of energy by conduction and convection. In addition, arc 
lamps operated on constant potential circuits are subject to still 
further losses owing to the necessity of series resistance or resistance 
as ballast. On constant-current circuits this ballast is unnecessary. 
The amount of loss in cases where lamps are operated on constant 
potential varies greatly in practice. As a rule the series resistance 
is not the least which would give stability, but is also made use 
of to adapt the are to the existing supply voltage. Thus, in the 
case of a carbon arc operating on a 110-volt circuit, more than half 
the total energy is wasted in resistance. On account of the un- 
certainty of this loss and of the entire absence of it in constant- 
current circuits no account is taken of it in the data on efficiency 
given below. | 

With regard to the conduction and convection losses no data ex- 
ist, so far as I know, which permit an accurate estimate of the 
magnitude of these losses. It is a well-known fact that the efficiency 
is increased by diminishing the thickness of the carbons, but the 
percentage loss through thermal conduction by the electrodes has 
probably never been determined. The fact that an arc is in contact 
with the air at atmospheric pressure would tend to make the loss 
by air conduction and convection great, but doubtless such an effect 
is much reduced by the fact that the radiation is proportional to a 
higher power of the temperature than the air loss probably is, so 
that in percentage it probably is much less than the loss with the 
Nernst lamp, for example. 

Owing to the complicated nature of the production of ight from 
the various arc lamps the available data do not permit the exact 
analysis of the energy relations such as is possible, for example, in 
the incandescent lamp. Thus we have seen that the temperature 
of the crater of a direct-current open carbon arc, which is chiefly 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES 63 


responsible for the efficiency of this type of arc lamp, is approxi- 
mately 3800°-4000° Abs. A black body at this temperature, with 
no losses by conduction, convection, ete., would be about 8 or 10 
per cent as efficient as the most efficient monochromatic source. 
Compared with this value the open carbon arc has an efficiency “ 
of only about 2 per cent. In other words, if there were no losses 
except in the infra-red radiation from a black body at 3800° Abs., 


the ratio (4 = +) of the luminous flux to the power input 
would be approximately 70 or 80 lumens per watt, whereas for the 


? 


direct-current carbon open arc the ratio + equals approximately 12 


or 15 lumens per watt. It must be borne in mind, however, that al- 
though the crater of the anode is at 3800°-4000° Abs., other parts of 
the anode near the tip, as well as the tip of the cathode, are radi- 
ating at much lower and hence much less efficient temperatures, 
so that a relatively large proportion of the supplied energy is 
radiated in the infra-red, compared with the radiation from carbon 
at 3800°-4000° Abs. | 


Similarly, whereas for carbon at 3800°-4000° Abs. the ratio a 


would be approximately 0.30, the actual values found for various 
types of carbon arcs by Marks and by Nakano range from 0.08 
to 0.17. | 

The efficiency of the alternating-current arc is roughly one-half 
to three-quarters that. of the direct-current arc, whereas with 
flaming and luminous arcs efficiencies from three to five times that 
of the direct-current carbon arc have been obtained. T’hus luminous 
efficiencies of 40-60 lumens per applied watt are found. Still higher 
efficiencies are possible in the use of electrodes exhibiting more pro- 
nounced selectivity in the visible spectrum. 

Under the head of arc lamps should properly come the mercury 
are, or mercury-vapor lamp, as it is more commonly called, but since 
in practice it is considered as a distinct type of lamp, and moreover 
resembles in some ways the vacuum-tube lamps, it will be consid- 
ered in a separate section devoted to these two types of illuminants. 


4. The Physics of Low-Pressure Arcs and Vacuum Tubes 


The mercury-vapor lamp” is an are at low pressure, and is to be 
distinguished physically from the nitrogen, carbon dioxide and other 
vacuum-tube sources with which, from its appearance, it might be 


64 ILLUMINATING ENGINEERING 


confused. The chief point of resemblance between the two types 
of lamps is that in each case the light is emitted by a luminescent 
gas or vapor at pressures considerably below that of the atmosphere. 
The distinguishing characteristic is the process by which the dis- 
charge through the tube takes place, with its effect on the nature of 
the radiation emitted. In the low-pressure mercury are the con- 
ducting material is mercury vapor supplied by the hot, mercury 
cathode, and the character of the hght is given by the emission 
spectrum of mercury vapor, just as in the luminous arc the cathode 
material enters the arc and determines the character of the radia- 
tion. The difference between the mercury arc and the ordinary 
luminous are is mainly one of pressure of the surrounding gas. 

In an ordinary vacuum-tube discharge, on the other hand, the 
conducting material is the gas between the electrodes, air, nitrogen, 
carbon dioxide, ete., and the character of the radiation depends 
on the emission spectra of these gases. The material of which the 
electrodes are composed plays no large part in determining the char- 
acter of the light emission. 

Considering first the low-pressure arc, as in the mercury-vapor 
lamp, the phenomena exhibited are in general the same as those 
presented in the discussion of ordinary ares. There is the same fall 
of potential ® at the anode and at the cathode, but owing to the low 
pressure in the tube the conductivity of the mercury vapor is much 
greater, permitting, or even necessitating, a much longer are for 
high efficiency. 

The temperature” of the mercury are in a glass tube is ap- 
parently quite low, and the explanation of the efficiency is ordinarily 
ascribed to luminescence, with a relatively large part of the energy 
in the visible spectrum. The efficiency” of the arc, as ordinarily 
operated, is variously given as ranging between 12 and 24 lumens 
per applied watt, corresponding to 0.5-1.0 watt per mean spherical 
candle. By using quartz instead of glass it is possible to operate 
the lamp with a much higher current density and greatly increased 
efficiency.” It is probable that an efficiency of about 50 or 60 
lumens per watt, corresponding to 0.20 or 0.25 watt per mean 
spherical candle, may be reached in the case of the quartz arc. It 
is believed, in this case that at high temperatures pure temperature 
radiation of increasing efficiency supplements the decreasing effi- 
ciency of the luminescent radiation. 


1 


PHYSICAL CHARACTERISTICS OF LUMINOUS SouRCES 65 


Data as to the losses by conduction, convection, etc., and as to the 
ratio ( z) of the energy in the visible spectrum to the total energy 


emitted are somewhat meager and indefinite. Lux™ finds for the 
Uviol (special kind of glass) and quartz lamps the values 0.058 
and 0.176, respectively, for the ratio . Moreover, for a Uviol 
lamp operating at about 0.65 watt per mean spherical candle, he 
finds that about one-half the total energy supplied to the lamp is 
radiated, the other half being dissipated at the electrodes, and by 
conduction and convection. 

In the mercury arc, as in other arcs, the material of one or both 
of the electrodes determines the character of the light emission, 
whereas in an ordinary vacuum-tube discharge the nature of the 
gas between the electrodes determines the spectrum, modified, how- 
ever, by such conditions” as pressure, potential gradient, ete. In 
the arc, after the gap between the electrodes is bridged, i. e., after 
the are is “struck,” the supply of “ions” or carriers of electricity 
is furnished by the negative electrode, and the conduction of cur- 
rent is continuous. The fall of potential at the electrodes of a 
vacuum tube is always very high, of the order of magnitude of 
several hundred or a thousand volts, so that the applied voltage 
must always be great. The fall in potential per centimeter length 
of tube is small compared with the fall of potential at the electrodes, 
and consequently very long tubes must be used in order that a 
moderate amount of the supplied energy may be radiated by the 
gas rather than practically all lost at the electrodes. ‘The con- 
ductivity of the gas varies with the pressure,” reaching a maximum 
at pressures of the order of magnitude of tenths of a millimeter 
of mercury. 

As in the mercury arc, the radiation is considered to be electro- 
luminescence. The efficiency depends on the distribution of energy 
in the emission spectrum, which varies from gas to gas. Angstrom” 
found for nitrogen a maximum of 91 per cent of the radiated en- 
ergy lying in the visible spectrum, 69 per cent for carbon dioxide, 
and 60 per cent for hydrogen. Commercial installations, as in the 
Moore tubes, have an efficiency * of 5 or 6 lumens per watt for the 
nitrogen tube, and about one-third or one-quarter that value for 
the carbon-dioxide tube. The great discrepancy between the lumi- 
nous efficiency of the radiation, and the actual luminous efficiency 


66 ILLUMINATING ENGINEERING 


of the lamp is to be ascribed to losses, of which those at the elec- 
trodes are by far the largest. Owing to the comparatively low tem- 
perature of the tube the conduction and convection losses are 
relatively small. 


5. The Physics of Open Flames and of the Incandescent Mantle 


The incandescent mantle lamp presents some of the most diffi- 
cult problems in the physics of luminous sources. In addition to 
the problems connected with the mantle itself are those of the 
Bunsen flame, and these latter are so intimately interwoven with 
the inter-molecular, or so-called chemical processes in the flame, 
that it is impossible to undertake a complete discussion of the flame 
in a lecture of this nature. And yet there are certain physical 
properties of flame sources which must be mentioned briefly as 
auxiliary to a consideration of the incandescent mantle. 

The open luminous flame was the earliest form in which gas 
was used as an illuminant, but the physics as well as the chemistry 
of the open flame has been the subject of much dispute, even in 
recent years. Various theories of the chemical transformations 
within the flame have been proposed with accompanying explana- 
tions of the light-giving properties of the flame.” The ultimate 
source of energy is chemical, but it has been a mooted question 
whether the radiation from the flame is dependent solely on the 
temperature or is due, at least in part, to chemi-luminescence. Ac- 
cording to the theory generally accepted at the present time the 
light from the open luminous flame is due to the temperature 
radiation from finely divided carbon particles heated to incan- 
descence by conduction from the hot gases of the flame. The 
spectral distribution” of the radiation is that which would be 
emitted by carbon at a temperature well within the accepted limits 
of temperature of the luminous zone, viz., 1500° Abs., at the 
beginning of the luminous zone and 2100° Abs. at the outer zone 
of complete combustion. ‘The luminosity of the flame will depend 
on the number of carbon particles present, and the temperature 
which they attain. The causes which tend to increase or decrease 
the luminosity of flames may therefore be divided into two classes, 
(1) those that affect the formation and quantity of the carbon, 
and (2) those that determine the temperature. 

The efficiency * of the open flame, considered from a physical 
standpoint is very low, but it is necessary to keep in mind the 


PHysIcAL CHARACTERISTICS OF Luminous SourcEs 67 


essential difference between the conditions prevailing in the pro- 
duction of light in the open flame and in the electric incandescent 
lamp, for example. In the former the chemical transformations 
with the generation of heat take place in the flame itself, and it is 
difficult to separate the efficiency of the heat production from that 
of the incandescent carbon particles rendered luminous by the heat. 
In the case of the electric incandescent lamp the chemical trans- 
formations, with the resultant generation of heat, take place under 
the boiler where the adduced gas burns (supposing a gas engine), 
and there are large heat losses even in the most efficient systems. 
Moreover, there is a second loss when the heat energy is trans- 
formed into electrical energy which must also be considered. It 
is unquestionably true, however, that the efficiency of the open 
luminous flame, even in its most efficient form in the regenerative 
burner, is still very low. Owing to the large conduction and con- 
vection losses the heat available for rendering incandescent the 
carbon particles is not large, and the radiant efficiency of these, 
because of the relatively low temperature,” is comparatively small. 

The open luminous flame has been very generally supplanted by 
the incandescent mantle, heated in a Bunsen flame. In the latter, 
which is particularly non-luminous, a mixture of gas and air is 
burned with the result that a more complete combustion takes place 
in the body of the flame. The temperature “ of that portion of the 
flame between the slightly luminous bluish-green surface of the 
inner zone and the outer limits of the outer zone ranges from about 
1800° Abs. at the inner zone to about 2000°-2150° Abs. at the 
outer zone. Although the maximum temperature of the Bunsen 
flame is perhaps but slightly if any higher than that of the open 
luminous flame, the average temperature of a large portion of the 
former is much greater than that of the latter, and the temperature 
to which finely divided solids placed in the Bunsen flame may be 
raised is much higher than any temperature available with the 
luminous flame. 

Coming now to the incandescent mantle in its most common 
form, consisting principally of the oxides of thorium and cerium, 
various hypotheses have been proposed to account for its high 
luminous efficiency. Following numerous attempts at the use of 
metallic mantles, such as the platinum gauze of Gillard,” and in 
one or two cases of mantles of infusible oxides, as the basket mantle 
exhibited by Claymond in 1880,” and the Fahnehjelm “ comb pat- 


68 ILLUMINATING ENGINEERING 


ented in 1885, Auer von Welsbach brought out his first mantle in 
1886." In his original patent application Auer mentioned various 
rare earths as particularly useful in securing light of various hues. 
Subsequently,” the mantle of approximately 99 per cent thoria and 
1 per cent ceria as used to-day was developed. 

Many years before the introduction of the Auer mantle the re- 
markable properties of certain of the rare earths when heated to 
incandescence were known. Bunsen in 1864” discovered that 
didymium earth when heated gives not only a continuous spectrum, 
but also superimposed bright bands. Bahr in 1865 “ found a similar 
phenomenon in the case of erbium earth. Bahr and Bunsen” con- 
jointly in 1866 made a further careful study of erbium oxide and 
came to the conclusion that the bright bands were emitted by the 
solid and not its vapor. Higginson, 1870,” confirmed these con- 
clusions, investigating besides erbium a large number of other 
materials, and found these bright lines and bands in the sare of 
lime, magnesia, etc. 

The practical use of Auer mantles stimulated further research 
into the properties of the rare earths. In 1891 Haitinger * studied 
neodymium and praeseodymium, using mantles saturated with the 
nitrate solutions. He found that pure neodymium shows the phe- 
nomenon very weakly and praeseodymium not at all, but that the 
addition of 1 per cent or less of aluminum oxide brings out the 
bright bands in both cases. The marked effects produced by the 
addition of small quantities of one earth to another in greatly in- 
creasing the luminous radiation have led to the widely different 
views that have been taken in explanation of the efficiency of the 
incandescent mantle. 

Without attempting a chronological treatment of the suggested 
hypotheses, it is interesting and important to mention briefly some 
of the theories“ that have been proposed, principally because at 
the present time no one theory is universally accepted. One of the 
earliest theories accounted for the high efficiency on the basis of 
phosphorescence, and there are those of the present time who hold 
that, although temperature radiation enters, the peculiarly high 
efficiency is to be ascribed to some form of luminescence. For the 
most part, however, the theories have been based on temperature 
radiation, but the variations have arisen in attempting to explain 
the observed phenomena on this basis. According to one theory 
which held sway for a time the high temperature was produced 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES 69 


locally by a catalytic action of the particles of the mixture of ceria 
and thoria composing the mantles. Even at the present time 
catalysis in one form or another is suggested as the cause of the 
high efficiency. 

The most probable theory, as accepted at the present time, was 
first proposed by Nernst and Bose, and afterward further elaborated 
by Féry. It is based on pure temperature radiation with selective 
emission, and suggests an explanation of the peculiar effects of 
mixtures which have made the problem so difficult of solution. 
Before outlining the theory the principal facts which seem to be 
fairly well established should first be presented. These facts are: 
(1) The temperature in the ordinary Bunsen flame probably does 
not exceed 2120° or 2140° Abs. at the region of highest tempera- 
ture, and consequently could not account for the high-luminous 
efficiency of the mantie if the latter radiated as a black body; (2) 
the spectra of the rare-earth oxides are in general peculiar in ex- 
hibiting banded spectra; (3) when a small quantity of certain of 
the rare-earth oxides, as ceria, is intimately mixed with some other 
such rare-earth oxide as thoria, and the mixture in a finely divided 
state, as in the incandescent mantle, is heated in a Bunsen flame, 
the mixture has a much higher luminosity than either constituent 
separately; (4) the luminosity of the mixture of ceria and thoria 
depends greatly on the proportions of the two constituents present. 
As the ceria is increased from 0 per cent to 1 per cent the luminos- 
ity rises to about 10 or 15 times its initial value, but rapidly de- 
creases again as the proportion of ceria is increased beyond 1 per 
cent; (5) the pure thoria mantle is probably at a temperature be- 
tween 100° and 150° lower than that of the flame, and the addi- 
tion of ceria causes a still further decrease; (6) thoria has a low 
emissivity, and no favorable selective emission in the visible spec- 
trum, whereas ceria has a much higher emissivity and pronounced 
selectivity in the visible spectrum. 

The theory most generally accepted as accounting for the phe- 
nomena exhibited by the mantle with varying proportions of thoria 
and ceria depends on the radiating properties of the two substances 
as given in (6). The mantle of pure thoria, owing to its low emis- 
sivity, assumes a temperature but slightly lower than that of the 
flame, but its luminosity is not great because the temperature is 
not very high, and thoria shows no favorable selective emission in 
the visible. The introduction of a very small quantity of ceria 


70 ILLUMINATING ENGINEERING 


lowers the temperature slightly because of the greater emissivity 
of ceria, but this decrease in temperature is much more than com- 
pensated for by the pronounced selective emission of the ceria in 
the visible spectrum. The integral effect therefore is to increase 
greatly the luminosity of the mantle. As the proportion of ceria is 
increased the luminosity constantly rises until the composition of 
the mantle is 99 per cent thoria and 1 per cent ceria, when the 
maximum luminosity is obtained. Further increments of ceria 
produce decreases in the luminosity because, owing to the high 
emissivity of ceria, the temperature of the mantle drops so low that 
the selective emission of the ceria is no longer sufficient to com- 
pensate for the decrease in temperature. 

Although this theory is probably the one most generally accepted 
at present, it is still open to question, and certain facts point to 
the existence of catalysis, or luminescence, or perhaps both. The 
peculiar nature of the spectra of the rare earths makes the problem 
difficult, as ordinary optical pyrometry is likely to give quite er- 
roneous results. Thus the temperature of the mantle” has been 
variously estimated from 1920° to 2470° Abs., and it is difficult 
to assign the correct value. The temperature of the Bunsen flame 
is by no means definitely established, and even if it were there 
would still be difficulties in arriving at the temperature of the 
mantle. ‘Those physicists who subscribe to the catalytic theory 
would see no objection to assigning to the mantle a temperature in 
excess of that of the flame. If there is no excess temperature, the 
question still remains as to what extent the temperature of the 
mantle is lower than that of the flame. The use of very small 
thermo-couples by White and Travers has led to the value 2020° 
to 2120° Abs. as the maximum temperature of the Bunsen flame, 
and the same method applied to the mantle has indicated tempera- 
tures 100° or 150° lower. The excessively high values that have 
been suggested have been obtained from the use of optical methods 
which are subject to large errors in cases of such selective radiation 
as that exhibited by the Auer mantle. 

In a similar way there is difficulty in determining the luminous 
efficiency of the incandescent mantle. White and Russell give as 
the consumption for the most efficient mantle containing 1 per cent 
cerium, 35 British thermal units per hour per candle (presumably 
measured horizontally). Since 1B.t.u. per hour equals approxi- 


mately aon calories per second, and 1 calory per second equals 


PHYSICAL CHARACTERISTICS OF LumINOUS SouRCcES er 


4.19 watts, the watts per mean spherical candle (taking the spherical 
25 X 250 _, 4.19 
60X60 * 0.88 
=11 watts per mean spherical candle, or approximately 1.1 lumens 
per watt. According to Fulweiler the most efficient incandescent 
mantle can be operated at 20 B. t. u. per candle, which would reduce 
the above values to approximately 6 watts per mean spherical candle, 
or about 2 lumens per watt. Lux gives for a mantle containing 
0.8 per cent cerium practically the same values as those found by 
White and Russell for a 1 per cent cerium mantle, and for a mantle 
containing 0.1 per cent cerium he finds an efficiency of about three- 
fifths that of the 0.8 per cent cerium mantle. 

As stated in an earlier paragraph regarding flames, the efficiency 
obtained as the ratio of the light produced to the heat energy sup- 
plied is not entirely comparable with the efficiency derived for an 
electric incandescent lamp by dividing the lumens emitted by the 
watts supplied, because in the generation of the electric power the 
efficiency of heat transformation is not 100 per cent. In a similar 
way the values given for the incandescent mantle are not com- 
parable with those ordinarily given for electric lamps. If the 
analysis were carried back to the coal in each case a more accurate 
comparison could be made, but such an analysis is beyond the scope 
of this paper. 


reduction factor equal to 0.88 as given by Lux) are 





The ratio z , 1. e., the ratio of energy radiated in the visible 


spectrum to total energy radiated has been found by White and 
Travers“ to be 0.045, being quite close to the value 0.05 for the 
Nernst, as obtained from several determinations. The ratio of L 
to the total energy supplied is given by Lux” as 0.005, indicating 
that only about one-tenth the energy supphed is radiated by the 
mantle. The remainder must be lost by conduction and convection. 
These figures are given merely to show the order of magnitude of 
the various energy losses as far as the published results may be 
accredited. 

Although no attempt has been made to give numerical values for 
open gas flames of ordinary illuminating gas, it may be well to 
mention briefly some of the characteristics of acetylene. Various 
values have been assigned for the temperature” of the acetylene 
flame, but it is probable that the temperature is not far from 


L 49 


2300° Abs. For R the average of several determinations bv 


72 ILLUMINATING ENGINEERING 
{ 

Angstrém, Nichols and Coblentz, and Stewart, is about 0.045, the 
same as that given for the incandescent mantle. Similarly for the 
efficiency ” Liebenthal quotes for ordinary burners an average spe- 
cific consumption of 1.1 liters of gas per candle-hour with a possible 
minimum of 0.65 liter per candle-hour. ‘Taking for acetylene the 
heating value given by Morehead” these figures lead to an average 
specific consumption of 19.3 watts per mean spherical candle with 
a minimum specific consumption of 11.6 watts per candle, corre- 
sponding to 0.65 and 1.1 lumens per watt, respectively. Lux gives 
for acetylene the specific consumption of 17.7 watts per candle, 
corresponding to an efficiency of 0.7 lumen per watt. 

It is to be borne in mind that in all discussions of flames and 
mantles large discrepancies may arise owing to the nature of the 
burner used, or to the exact nature of the gas, or to the regulation 
of the gas in the burner. For these reasons only approximate 
values are attempted. 


6. The Nernst Glower 


Closely akin to the incandescent mantle and suggested by it, 
the Nernst glower nevertheless stands out uniquely from any other 
practical illuminant. like the incandescent mantle it is com- 
posed of oxides of the rare earths, but unlike the mantle it is heated 
to incandescence by the passage of an electric current. The Nernst 
glower is what is known as a solid electrolyte, i. e., a substance 
which conducts electrolytically. (as distinguished from metallically) 
when at a sufficiently high temperature. At ordinary temperatures 
it is an electric insulator. 

The work of Nernst, which led to a patent application in 1897, 
was probably anticipated to a certain extent by Jablochkoff,” who 
in 1879 made an electric lamp whose radiating body was made of 
a small plate of kaolin, a portion of which was.rendered incan- 
descent by the spark-discharge current of an induction coil. The 
detailed patents of Nernst, given out in 1901, covered a number of 
combinations, involving the oxides of zirconium, thorium, cerium, 
erbium and yttrium, which may be used to make satisfactory 
glowers. According to an analysis given by Beebe” several years 
ago (1905), the regular commercial glower as manufactured in this 
country consists normally of 85 per cent of zirconia and 15 per cent 
of yttria, but from other descriptions that have been given it seems 
probable that erbia, thoria and ceria have at times been included. 


PuystcaAL CHARACTERISTICS OF Luminous SourcEs 13 


It is an interesting fact that the pure oxides are not as satisfactory 
as are mixtures of two or more oxides, either from the standpoint of 
electrical conductivity or luminous radiation. 

The energy relations in the Nernst glower are still to a great 
extent a matter of speculation. The destructive electro-chemical 
decomposition at the electrodes which accompanies electrolytic con- 
duction is supposed in the case of the Nernst glower to be counter- 
acted by the oxidizing action of the air. The glower consequently 
will not operate in a vacuum, and is hence subject to losses of 
energy by thermal conduction and convection of the air. Further- 
more, it has been suggested“ that since the atmosphere surround- 
ing the glower is ionized and there is present a very appreciable 
potential gradient some of the energy supplied the lamp may be 
conducted electrically through the surrounding air and not through 
the glower body. 

Regarding the losses through thermal conduction and convection 
in the air there are several published estimates.” According to 
Hartmann these losses amount to anywhere from 5 per cent to 70 
per cent, depending upon the assumption on which the estimate 
is made. Lux gives the loss as about 30 per cent, and Leimbach 
as approximately 50 per cent. A recent experiment made to indi- 
cate roughly the order of magnitude of these losses gave as a result 
a loss of approximately 50 per cent to within +10 per cent. From 
the measured losses in a platinum lamp when burning in air it 
would scarcely seem probable, notwithstanding the distinct char- 
acteristics of the two filaments, that the losses in the Nernst should 
be as low as 5 per cent or 10 per cent. 

Due to the pronounced negative temperature coefficient for the 
material of the glower at ordinary temperature, it is necessary to 
place in series with the glower a ballast resistance. The magnitude 
of this temperature coefficient is evidenced by the fact that for a 
110-volt, 44-watt lamp the resistance of the glower which is 320 
ohms at normal voltage (110 volts), drops to 240 ohms at 125 
volts, and rises to 600 ohms at 92 volts.” The necessary ballast, for 
which is chosen a material with high-positive temperature coeffi- 
cient, must have a resistance such that at normal burning 10 per 
cent of the supplied energy is lost in the ballast. There is another 
known loss of 2 per cent, which arises from the necessity of a 
magnetic cut-out to throw the heating coil out of circuit when the 
glower begins to conduct.” 


"4. ILLUMINATING ENGINEERING 


The question of losses is intimately connected with that of the 
explanation of the efficiency “ of the Nernst, for which values rang- 
ing from 2.0 to 3.0 watts per mean spherical candle have been given. 
It is probable that the average value lies between 2.4 and 2.8 watts 
per mean spherical candle corresponding to 4 or 5 lumens per ap- 
plied watt. If there is any large loss of energy by conduction or 
convection, this efficiency could only result from moderately high 
temperature or markedly selective emission. ‘The estimates of 
temperature” that have been given range from 1800° Abs., made 
from extrapolated thermo-couple measurements, to 2450° Abs., de- 
termined by optical methods. It is quite probable that the true 
temperature is at least above 2000° Abs., and hence several hundred 
degrees higher than that of the incandescent mantle. 

To what extent selective emission determines the efficiency is 
not known. In the discussion of the mantle it was seen that the 
rare-earth oxides frequently exhibit selectivity to the extent of 
pronounced bands, particularly at low temperatures and in a finely 
divided condition. Such a banded spectrum has been observed for 
the Nernst glower, both in the visible and infra-red regions at 
abnormally low temperatures, but the bands disappear at higher 
temperatures so that in the neighborhood of the temperature of 
normal operation the spectrum is practically continuous. At this 
temperature, if there is any selective emission, at least in the visible 
spectrum, it is of the nature of that found for metals, being merely 
an exaggerated relative emission in the shorter wave-lengths, as 
compared with the radiation ofa black body at the same temperature. 


Various estimates have been made of the ratio (= | of the 
energy radiated per second in the visible spectrum to the total 
emission per second.” Ingersoll, using Angstrém’s method, obtained 
for a 110-volt, 89-watt glower, 0.046; Drude quotes a value of 
0.065 for a glower at 1 watt per hefner, which perhaps corresponds 
to a slightly higher temperature than normal operation. Coblentz, 
by integration of an energy curve obtained 0.055 for a filament at 
83 watts (presumably 88 watts normal). It is probable that 
ie =0.05 expresses approximately the relative amount of energy 
radiated in the visible spectrum between A=0.38 » and A=0.76 up. 
Without a knowledge of the losses in the glower it is impossible to 
compute the ratio 1 of the energy radiated in the visible spectrum 


Q 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES G5 


per second to the power supplied to the lamp. If we assume the 
conduction and convection losses to be 50 per cent, as indicated by 


the experiment to which reference was made, then = 0.50, and 
L L R 
hence —- = —- x — =0.025. On the basis of the same assump- 
Dina 0 ; 
tion, in conjunction with the average value of 2.6 watts of power 
supplied per mean spherical candle, or 4.8 lumens per watt, in a 
normal lamp, we obtain the value of + , the ratio of luminous flux 
to total energy radiated as about 9.6 lumens per watt, which would 
be obtained from a black body at 2200°-2300° Abs. From this it 
would seem that the temperature of the Nernst is either quite 
high or else that there is selective emission to partly account for 
the efficiency. 
The same result can be arrived at from Drysdale’s” value of the 


p 


so-called mechanical equivalent (--) for the Nernst, which is 


150 lumens per watt. Taking io 
lows that Ra = < x 7 =150x0.05=7.5 lumens per watt radi- 
ated. Indeed, the extension of this method leads indirectly to the 
energy emission in the Nernst. For, if the efficiency is 7.5 lumens 


=0.05, as given above, it fol- 


per watt radiated (+), and 4.8 lumens per watt supplied a) 
it would follow that 5 = 0.64, and so the loss (Q—R) is approxi- 


mately 36 per cent of the power supplied. 


7. The Physics of the Fire-Fly and Other Inght-Producing 
Organisms 

Although the fire-fly can scarcely be considered as a commercial 
illuminant, its interest and the attention which it has received 
merits its brief consideration here. It is peculiarly fitting that 
this natural illuminant should be discussed at the end of the series 
of human attempts at light production because, from the stand- 
point of radiant efficiency, it surpasses any other known source. 
In the first lecture we found that if all the energy supplied to a 
lamp were radiated, and if all the radiant energy lay at that wave- 


76 ILLUMINATING ENGINEERING 


length in the visible spectrum to which the average human eye is 
most sensitive, the highest possible efficiency would be obtained. 

In the fire-fly we have practically an exemplification of at least 
one of these requirements.” According to the best information at 
present it would appear as though, from the standpoint of radia- 
tion, the efficiency of the fire-fly is almost as great (estimated at 
96-97 per cent) /as that of the most efficient radiator possible. 
Unfortunately, we do not know what chemical and biological trans- 
formations occur in the process of “ glowing,’ and without this 
knowledge we can form no idea of the real efficiency of trans- 
formation. 

The process of light production in the fire-fly is called lumines- 
cence, and seems to depend on the presence of oxygen and water. 
Other living organisms, such as glow worms, certain bacteria and 
numerous fishes, exhibit the property of light production, but our 
knowledge of these at present is quite meager. Further investi- 
gation of these natural lamps may disclose processes of light pro- 
duction which could with profit be copied by man in the construction 
of artificial sources. 


8. The Distribution of Energy in the Spectra of the Various 
Luminous Sources 


One of the most interesting of the physical characteristics of 
luminous sources is the distribution of energy in their spectra. The 


spectral distribution determines the ratio (=) of the energy 
radiated per second in the visible to that radiated per second in the 
complete spectrum, and also the ratio (£) giving the photo- 
metric value of the visible radiant energy. Eliminating conduc- 
tion, convection and other incidental losses, the energy distribution 
determines the commercial efficiency of practical sources, accounts 
for the quality of the composite light, and explains the appearance 
of colored objects illuminated by it. We are interested to know 
whether the spectrum of a source is continuous, discontinuous or 
banded; what proportion of the energy is in the visible spectrum, 
and whether in cases of continuous spectra the distribution is that 
of black-body radiation at some temperature or distinctly different 
from it owing to pronounced selectivity. 

It is ordinarily considered that gases and liquids when incan- 


PHYSICAL CHARACTERISTICS OF LumMINOUS SOURCES rind 


descent emit discontinuous spectra, but that solids in general emit 
continuous spectra in which the energy is distributed very much 
the same as in the spectrum of a black body. But even for luminous 
gases and vapors certain distinctions must be made in the light of 
recent experiment and theory. The bright-line spectrum, as in 
the spectrum of sodium when common salt is heated in a Bunsen 
flame, has generally been considered as intimately connected with 
some chemical reaction, in the course of which the sodium atoms 
are brought into a radiating state, which cannot be reproduced by 
mere heating of sodium vapor. When gases or vapors are heated, 
it has usually been agreed that only banded spectra are obtained, 
except when the temperature is so high, as in the quartz-mercury 
arc, that there is a continuous spectrum as background to the bright 
lines. The bright lines are obtained only when chemical or elec- 
trical excitation is employed, and not when the gas or vapor is 
merely heated. Some recent experiments by Bauer seem to indi- 
cate an exception to this rule that bright-line spectra are always 
associated with so-called luminescence, but this work is too recent 
to justify a,reversal of opinion in this regard at the present time.” 

For solids which radiate approximately as a black body the lumi- 
nous efficiency increases rapidly with the temperature, and it is to 
the temperature influenced to some extent probably in all cases, 
and to a very considerable extent in some cases by selectivity in the 
emission, that the luminous efficiency of many sources is to be 
ascribed. There is a class of solids, however, illustrated by the 
rare-earth oxides of the incandescent mantle and the Nernst glower 
which, though solid, exhibit at least at low temperatures, peculiar 
banded spectra superimposed upon a continuous background. 

It is perhaps not appropriate to discuss in these lectures theories 
of spectral energy distribution, however attractive such a com- 
parative discussion might seem in the light of the varied spectral 
phenomena exhibited by the different commercial light sources. The 
presentation of the spectra of all the sources in one section, how- 
ever, does not contemplate such a discussion, but is intended rather 
to give a better comparative idea. 

In Figure 6 are plotted the spectral energy curves in the visible 
region for various common light sources. These curves show the 
absolute distribution of energy for each source, but not the absolute 
amount of energy radiated per unit area, being plotted in arbitrary 
units so chosen that the ordinate at A}=0.59 w is the same for all. 


78 ILLUMINATING HNGINEERING 

The curves were taken from the best published values” in most 
cases, but were partly determined by the author to fill in gaps in 
the literature of the subject. The variations in the curves obtained 
by different observers for some of the sources were found to be 
strikingly large, owing partly to the conditions of the experiment 

































































N 





SR\ SNES 
BERSNG 
Sl eae To 


Woh Vesa eos eaC eee a 
el: AE 2 ype Gis ae | tee eae Te 





oO 
Taper Y) 
mm ZA _| 
at |_| 
mm 
lane 
eee |_| 
me ra 
LZ ies 
TEER. raed bala ENS MUU IGG? 
4 5 60 65 .70 
Alte aur 


Fig. 6.—Energy Curves for Different Sources in the Visible Spectrum. 


and partly to variations in the exact nature of the illuminant. In 
such cases what seemed to be the best average value was taken. 
Spectral energy curves in the visible spectrum are given for the 
following light sources: ordinary carbon lamp at 3.1 watts per 
mean horizontal candle, tantalum lamp at 2 watts per mean hori- 
zontal candle, and tungsten lamp at 1.25 watts per mean horizontal 
candle; Welsbach regular commercial upright mantle (average of 


PHYSICAL CHARACTERISTICS OF LuMINOUS SOURCES 79 


various specimens), Nernst glower (average of various types), and 
acetylene (average of various determinations with different burners, 
and under different conditions). In every case the spectrum is con- 
tinuous. But, although the spectra are continuous, the distribu- 
tion is not in every case that which could be obtained from a black 
body at the proper temperature. Thus the incandescent mantle 
shows evidence of a pronounced selectivity in the green. 

The spectrum of an incandescent mantle depends greatly on the 
composition of the mantle and on the temperature to which it is 
heated in the Bunsen flame, which latter depends on the heating 
value and composition of the gas used, and on the adjustment of 
the flame. The spectrum of acetylene depends on the burner and 
on the thickness of the flame, containing relatively more blue in 
the thin than in the thicker flames. It also depends to some extent 
on atmospheric conditions. Hence, the curves given for these 
sources, the incandescent mantle and acetylene, can only be con- 
sidered as representative of the general type of curve obtained. | 
Indeed, investigators frequently fail to give the exact specifications 
of the sources employed, making: accurate comparisons impossible. 
It would seem as though, despite the immense amount of work 
_done on commercial light sources, there is still lacking a compre- 
hensive comparative study of the exact spectral compositions of 
these sources under carefully defined conditions. 

It was hoped that a curve for the open are might be included, 
but when the literature was searched for data on the energy dis- 
tribution in the visible spectrum of the arc, very few curves were 
found, and these showed such enormous discrepancies that no value © 
could be attached to a mean curve derived from them. The curve 
is continuous in the visible spectrum with a superimposed band in 
the blue due to the arc flame. 

The spectra of the Moore tube, the mercury are and the various 
luminous and flaming arcs have not been given in Figure 6 because 
their spectra are discontinuous, consisting mainly of distinct bright 
lines, which it would be difficult to represent to scale of intensity. 
For such sources the most important facts for us to know are (1) 
how closely the bright lines occur, and (2) the integral color of 
the light, i. e., the color which a white surface would assume when 
illuminated by the hight. These questions will be discussed sub- 
sequently in considering the quality of the light from the various 
sources as determined by the use of the colorimeter. 


80 ILLUMINATING ENGINEERING 


Comparable in importance with the spectral energy distribution 
in the visible spectrum, that in the infra-red region demands our 
consideration.” In fact it is the relative amount of energy in the 
visible as compared with that in the infra-red, taken together with 
the distribution in the visible spectrum, which determines the 
candle-power for each watt radiated. The difficulties in the way 
of making accurate infra-red measurements are in some ways greater 
than those encountered in the visible spectrum, which no doubt 
explains the paucity of available data on infra-red energy curves. 
Coblentz gives as the infra-red curve for a tungsten lamp, pre- 






Pa A a ES Fs Te 
EEEEE EEE EEE EEE 
Bas PP 

| BRP: 














SUEY GA IED AMM 
Be ARERR SRNECeS Chee. 
a 


NAN A ae 
pA 




























EcEEEECEEE 
He 
sane 
ed 
Ae 
pa 
Kil 
Y| | lA 
aa Ac 
wa 
Z| 








Fig. 7.—a. Energy Curve for Tungsten Lamp at Normal Voltage. 
b. Energy Curve for a Black Body at 2200° Abs. 


sumably under normal conditions, that shown in Figure 7, curve 
“a”? In general form it resembles the energy curves of a black 
body, but differs somewhat from the latter. Thus, if we plotted 
the energy curves of a black body at such a temperature that the 
distribution in the visible spectrum was the same as that of the 
tungsten, the two curves would differ in the infra-red, that of the 
tungsten lying below that of the black body. Such a black-body 
curve is given in curve “b,”’ corresponding to a black body at 
2200° Abs., at which temperature the black body has approximately 
the same distribution of energy in the visible spectrum as the 
tungsten lamp at normal efficiency. ‘The two curves are plotted 


PHYSICAL CHARACTERISTICS oF LUMINOUS SOURCES 81 


to the same ordinate at the same wave-length of the visible spec- 
trum, say A=0.7 p. 

For the electric incandescent lamps at normal operation, curves 
somewhat similar to that of tungsten would be found. For carbon 
the curve would correspond more nearly to that of a black body, 
but the temperature of the black body for the same spectral distri- 
bution in the visible would be lower than that for tungsten. For 
osmium probably the greatest deviation from the black body would 
be found. 


i 
al 
rl 





ERs 
4 Bree rete etaya rr rrr 
Peeeietetererd ar legriteiorl fs fe Pt 





. eeteete eel ie heed te it | 
fe) 2 4 6 8 10 12 14 18 
Xt Ak, 
Fig. 8.—Energy Distribution for a Welsbach Mantle—According to 
Rubens. 


The infra-red curve for an incandescent mantle (composition 
99.2 per cent thoria, 0.8 per cent ceria) has been found by Rubens, 
by subtracting the radiation of the open Bunsen burner from the 
combined radiation of the burner and mantle. This curve is given 
in Figure 8. The peculiar broken form of the curve seems to 
be characteristic of the radiation from rare earths at moderate tem- 
peratures. The curve for the Nernst found by Coblentz (Figure 9) 
is quite smooth, although the glower, like the incandescent mantle, 
is composed of rare-earth oxides. But at lower temperatures the 
Nernst glower also shows evidence of a banded spectrum. Whether, 
in the case of the Nernst at normal operation, the smoothness is 
to be ascribed entirely to a higher temperature than the mantle, 
or to its more compact form, remains to be determined. 


82 ILLUMINATING ENGINEERING 


In Figure 10 is given the infra-red curve of acetylene as found 
by Stewart. For the same reasons as those given for omitting the 
visible spectra of the various flaming and luminous ares and vacuum- 
tube lamps, as the Moore tube, no attempt will be made to give 
here the infra-red energy curves of these sources. 






wie 
als ba hs a eli as eS ied hy Bele 
TT Pel Valet se a) ae ee 
‘ERED RMRERRMMARM Oe. 
UREBRERRRRRREMELES OL 
SVPRNNRNMIWERAAe eo 
eee 
PHA PDMS 
DRC CMBMRIMIEIGEIRI 
HRMMMRMMBRMEMT 
4— 
BRR is a 35) 









































BSTC MM Me ME MMPs 
SEH EE CECE EDN tet eas isle ae ea 
PLS ote 
EPEEEEEE SEEPS EEE 
SEES ECE EEC 


"Seo 








Fic. 9.—Energy Distribution of a 110-Volt Nernst Glower Operated at 
77.7 Watts—According to Coblentz. 


9. The Quality of the Light from the Various Luminous Sources 


Closely associated with the question of spectral energy distribu- 
tion is that practical one of the quality of the light from illumi- 
nants and the appearance of colored objects when illuminated by 
these illuminants, as explained at length in the first lecture. When 
the energy distribution in the visible spectrum is continuous and 
represented by a smooth curve, the integral color of a source, i. e., 
the color which a white surface illuminated by it assumes, is a fair 
indication of the variation in color values which will occur when 
the source is substituted for average daylight, taken as normal 
white light. But if the spectrum of an illuminant is discontinuous, 


PuHysicaAL CHARACTERISTICS OF Luminous SourcES 83 


composed of a number of distinct lines, the distribution of these 
lines, together with the integral color, must be examined. 

The integral color of the light from any source can readily be 
measured by determining the relative amounts of red, green and 
blue light which when mixed give a resultant color which matches 
in hue that from the source under investigation. Such measure- 


(2 SEs eae 
Mp po 
pee Mini leet | fo] | 

ve aol ES) 

US6 Raye ae eee 
Peete aereeiiel beret eb TL 

PERE EEE 























[ >) 


BEE EEE REE 
ho JS Sa ee Sane Ree 
SEE 
eee eee eter ep N ee a 
Seimei ore ttt tS 
eer ie eae tet lel eT | TAS ET TT 
eet ee eee AA 
Sipe seme ion eile eb poet fab Peleg 





rN) 








Ben img Ae 


Fig. 10.—Energy Distribution for Acetylene Flame—According to G. W. 
Stewart. 


ments carried out with the F. E. Ives colorimeter have been pub- 
lished by H. E. Ives” for a number of illuminants. These results 
are given in Table II. White light is taken as that emitted by a 
black body at 5000° Abs., for which the sensation values are red 
33.3 per cent, green 33.3 per cent and blue 33.3 per cent. The 
color values of the various illuminants are expressed in terms of 
red, green and blue sensations, such that the three values given 
add up to 100 per cent. 

From a consideration of this table it is seen that the carbon- 
dioxide vacuum tube approaches most nearly to average daylight. 


84. ILLUMINATING ENGINEERING 


Although the spectrum of the vacuum-tube source is always dis- 
continuous, the number of bright lines in the spectrum of carbon 
dioxide is very large, and the lines are distributed throughout the 
entire visible spectrum, being thus equivalent for practical purposes 
to a continuous spectrum. The other sources which show discon- 
tinuous spectra, as stated in the discussion of spectral energy dis- 
tributions, are the low-pressure mercury are and the ordinary lumi- 
nous and flaming arcs. In the case of the mercury arc the effect 
of the visible spectrum being composed of a few lines widely sep- 
arated is plainly shown in the unnatural appearance of certain 
colored objects illuminated by its light. 

One significant feature in regard to the integral color of hght 
sources is the relatively different impressions produced by two lights, 
each slightly different from average daylight, when the direction 
of the difference is one way or another. If the color of a light is 
approximately that which a black body gives at some temperature, 
it does not appear nearly so strikingly different from daylight, 
although the hue may be distinctly reddish, as a hght which differs 
from daylight in such a way as not to lie on the scale of color 
which a black body assumes as the temperature is varied. The 
explanation of this phenomenon comes rather within the province 
of physiological optics than that of physics. 


TABLE II 
CLASSIFICATION OF LIGHT SOURCES ACCORDING TO COLOR VALUES 


Sensation values. 


SEES Red. Green. Blue. 
1. Blacksbody>at/5000° "Abs? 7 7.022 33.3% 33.3% 33.3% 
2. Bluevsky ws. 46 the os ee eee 32.0 32.2 35.8 
3, Overcast sky! esac Lene ee SHGNiEE LE SRO 31.5 
4, Afternoon Aut 2 oa ban tae ee eee olan 3f.3 25.0 
GS, FLGINGT ny coe here ccs Giese ee 55.0 38.8 6.2 
6°31 we Dd. ‘e.vearbon Wamp ..'). eee dL 40.4 8.3 
1 Acetylene) 3363. ORastes 2 lekk eee ee 49.1 40.5 10.5 
S.« Tungsten ol 25 nWadid.. 26tt ). ae eee 48.7 40.5 10.9 
Dep NOTUS la css be Ee ec ee 49.2 40.7 a Fi 
10. Welsbach, 42 Oo" Cerlas wav. st eens 42.5 40.8 16.7 
11: “Welsbach "97%, ceria” <2... eae 45.5 42.0 12.5 
12.) Welsbach, 14496 ceriadea. Jia gee 47.2 41.8 14.0 
13. Direct’ current atemew....... eee 41.0 36.3 oouD 
1d) Mercury, ALC hese bes wetnke ee 29.0 30.3 40.7 
15. Yellow flame arc. ua0.. a... en eee 52.0 31.0 10.5 
16. Moore carbon dioxide tube......... 5a Ne 31.0 one 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES 85 


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PHyYsIcAL CHARACTERISTICS OF Luminous SourcEs 87 


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4 


Set ot ncaa 


mo 


88 


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. W. Huggins, Proc. Roy. Soc. 18, p. 546, 1870. See also: Phil. Mag. 


(4) 30, p. 30251570: 
L, Haitinger, Monats. f. Chemie, 12, p. 362, 1891. 
E. L. Nichols and B. W. Snow, Phil. Mag. 33, p. 19, 1892. 
Ch. St. John, Wied. Ann. 56, p. 433, 1895. 
C. Killing, Jour. f. Gasbeleuch., p. 697, 1896. 
V. B. Lewes, Jour. Gas Lt. (Lond.), p. 1104, 1896. 


45. 


46. 
47. 
48. 


49. 


50. 


dl. 
52. 


53. 


54. 
D5. 


PHYSICAL CHARACTERISTICS OF LUMINOUS SOURCES 


Drossbach, Jour. f. Gasbeleuch. 40, p. 174, 1897; 4/4, p. 352, 1898. 


H. Bunte, Ber. Chem. Gesell. 31, p. 5, 1897. 

Moschell, Zeit. f. Beleuch. 11, 1897. 

H. Le Chatelier et O. Boudouard, C. R. 126, p. 1861, 1898. 
Bei. zu den Ann. der Phys. 22, p. 313, 1898. 

A. A. Swinton, Proc. Roy. Soc. 65, p. 115, 1899. 

W. Nernst and E. Bose, Phys. Zeit. 1, p. 289, 1900. 

H. Thiele, Ber. Chem. Gesell. 33, p. 183, 1900. 

H. Kayser, Spectroscopie, 2, p. 161, 1902. 

C. Féry, C. R. 184, p. 977, 1902. 

M. Solomon, Nature, 67, p. 82, 1902. 

H. Bunte, Ber. Int. Cong. d. Chemie, Berlin, May, 1903. 
St. Clair Deville, C. R., 1903. 

H. Rubens, Phys. Zeit. 6, p. 790, 1905. 

J. Swinburne, Elec. (Lond.) 57, p. 744, 1906. 

H. Kayser, Spectroscopie, p. 452, 1906. 

Foix, C. R. 144, p. 685, 1907. 

R. J. Meyer and A. Auschiitz, Sci. Abs. 10 A, p. 588, 1907. 
Ill. Eng. (Lond.) 17, pp. 173 and 958, 1908. 

A. Simonini, Trans. Ill. Eng. Soc. 4, p. 647, 1909. 

H. Le Chatelier et O. Boudouard, C. R. 126, p. 1861, 1898. 


A, White and A. Travers, Jour. Soc. Chem. Ind. 27, p, 1012, 1902. 


Holborn u. Kurlbaum, Ann. der Phys. 10, p. 237, 1903. 
H. Rubens, Phys. Zeit. 7, p. 187, 1906. 
H. Rubens, Ann. der Phys. 20, p. 5738, 1906. 


-H. Lux, Zeit. f. Beleucht. 33, p. 375, 1909. 


89 


A. White and A. Travers, Jour. Soc. Chem. Ind. 21, p. 1012, 1902. 


H. Lux, Zeit. f. Beleucht. 33, p. 375, 1909. 
Le Chatelier, C. R. 121, p. 1144, 1895. 

V. Lewes, Chem. News, 71, p. 181, 1895. 
Smithells, Jour. Chem. Soc. 67, p. 1050, 1895. 
. L. Nichols, Phys. Rev. 10, p. 234, 1900. 

. Ladenburg, Phys. Zeit. 7, p. 697, 1906. 

. Nichols, Phys. Rev. 11, p. 215, 1900. 


Aha & 


8, p. 257, 1902. 
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. Stewart, Phys. Rev. 16, p. 126, 1903. 
. Liebenthal, Praktische Photometrie, p. 357, 1907. 
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J. Morehead, Acet. Jour. 1/, p. 261, 1910. 


Te 2 


_ Angstrém, Astrophys. Jour. 15, p. 223, 1902. See also Phys. Zeit. 


Bussman und Boehm, EHlek. Zeit. 24, p. 281, 1903. See also EH. De- 


Fodor, Sci. Abs. 2, p. 713, 1899. 
M. C. Beebe, Sci. Abs. B, 8, p. 398, 1905. 
Elec. World, 43, p. 981, 1904. 
H. N. Potter, Proc. Inter, Elec. Cong., St. Louis, 2, p. 852, 1904. 
AS, Wurtz, Trans fAul. E B..738iipreslip too. 
L. Hartman, Phys. Rev. 22, p. 353, 1906. 


90 


ILLUMINATING ENGINEERING 


H. Lux, Zeit. f. Beleuch., 1907. 
G. Leimbach, Zeit. f. wiss. Phot. 8, p. 395, 1910. 


56. F. Hirschauer, Elek. Zeit. 29, p. 87, 1908. 


57. 


58. 


59. 


W. Nernst and W. Wild, Zs. f. Elektrochem, 7, p. 373, 1900. 

Herzog, u. Feldman, Handbuch d. Elek. Beleuch., p. 70, 1907. 

W. Wedding, Elek. Zeit. 22, p. 631, 1901. 

Zeit. f. Instr. 23, p. 178, 1903. 

Elec. World, 43, p. 981, 1904. 

M. C. Beebe, Hlec. Rev. 46, p. 657, 1905. 

J. Herzog u. C. Feldmann, Handbuch der Elek. Beleuch., p. 144, 1907. 

O. Lummer u. E. Pringsheim, Verh. der Deutsch. Phys. Ges. 1, p. 
235, 1899. 

F. Kurlbaum und G. Schulze, Ber. der Deutsch. Phys. Ges. 1, p. 428, 
1908. 

L. R. Ingersoll, Phys. Rev. 17, p. 376, 1903. 

L. Hartman, Phys. Rev. 22, p. 353, 1906. 

Mendenhall and Ingersoll, Phys. Rev. 24, p. 230, 1907; 25, p. 12, 1907. 

W. Coblentz, Bul. Bur. of Stds. 4, p. 536, 1907. - 

W. Coblentz, Bul. Bur. of Stds. 5, p. 183,.1908. 


60. L. R. Ingersoll, Phys. Rev. 17, p. 371, 1903. 


61. 
62. 


63. 


64. 


65. 


Drude, Lehrbuch der Optik, p. 474, 1906. 

W. W. Coblentz, Bul. Bur. of Stds. 4, p. 553, 1907. 

W. W. Coblentz, Bul. Bur. of Stds. 5, p. 184, 1908. 

C. Drysdale, Ill. Eng. (Lond.) 7, p. 648, 1908. 

S. Langley and F. Very, Phil. Mag. 30, p. 260, 1890. 
Broomall, Sci. Amer. Nov. 5, 1898. 

H. E. Ives and W. Coblentz, Trans. Ill. Eng. Soc. 4, p. 657, 1909. 
A. Krug, Astrophys. Jour. 28, p. 300, 1908. 

M. E. Bauer, C. R. 130, p. 1747, 1910. 

E. Nichols and Franklin Amer. Jour. of Sci. 38, p. 100, 1889. 
¥. Gaud, C. R. 129, po759;/1899; 

Blaker, Phys. Rev. 73, p. 345, 1901. 

P. Vaillant, C. R. 142, p. 81, 1906. 

E. Nichols, Trans. Il]. Eng, Soc. 3, p. 322, 1908. 

H. Kayser, Spectroscopie, 3, p. 427, 1905. 

E. Kottgen, Ann. der Phys. 53, p. 801, 1894. 

Nernst and Bose, Phys. Zeit. 1, p. 289, 1900. 

H. E. Ives, Bul. Bur. of Stds. 6, p. 284, 1909. 

G. Stewart, Phys.. Rev. 16, p. 125, 1903. 

L. Hartman, Phys. Rev. 17, p. 65, 1903. 

E. Nichols, Phys. Rev. 30, p. 333, 1910. 

Kurlbaum und Schulze, Ber. d. Deutsch. Phys. Ges. 1, p. 428, 1903. 
S. P. Langley, Phil. Mag. 29, p. 52, 1890. . 

H. E. Ives, Trans. Il]. Eng. Soc. 5, p. 208, 1910. 

E. Nichols, Phys. Rev. 2, p. 260, 1894. 

E. P. Hyde, Trans. Ill. Eng. Soc. 4, p. 334, 1909. 

W. Coblentz, Bul. of Bur. of Stds. 5, p. 360, 1908. 

W. Coblentz, Jour. Frank. Inst. 170, p. 174, 1910. 


66. 


PHYSICAL CHARACTERISTICS oF LUMINOUS SOURCES ou 


Le Chatelier et Boudouard, C. R. 126, p. 1861, 1898. 

O. Lummer and G. Pringsheim, Verh. d. Deutsch. Phys. Ges. 1, p. 
235, 1899. 

Rubens, Phys. Zeit. 6, p. 790, 1905. 

Rubens, Phys. Zeit. 7, p. 186, 1906. 

W. Coblentz, Bul. Bur. of Stds. 6, p. 173, 1910. 

G. Stewart, Phys. Rev. 16, p. 125, 1903. 

W. Coblentz, Bul. Bur. of Stds. 4, p. 533, 1907. 

W. Coblentz, Bul. Bur. of Stds. 5, p. 184, 1908. 

E. Drew, Phys. Rev. 17, p. 321, 19038. 

W. Voege, Jour. f. Gas Beleuch. 48, p. 513, 1905. 

H. E. Ives, Trans. Ill. Eng. Soc. 5, p. 208, 1910. 

D. McF. Moore, Trans. Ill. Eng. Soc. 5, p. 209, 1910. 


















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III 
THE CHEMISTRY OF LUMINOUS SOURCES 


By Wiis R. WHITNEY 


CONTENTS 


Introduction. 
Peculiar position of the element carbon in almost all lighting systems. 
Carbon heated to luminescence in oil, illuminating gas and acetylene 
flame. 
Are lighting and incandescent lighting. 
Substitution of other materials for luminous carbon in flames. 
Drummond light. 
Welsbach mantle. 
Carbon arc lighting—History of. 
Electrochemistry of the arc. 
Combustion and electrical migration. 
Enclosed are and air control. 
Direct and alternating current arcs. 
Arcs of other material than carbon. 
Solids heated by arc. 
Non-carbon arcs. 
Iron, magnetite, titanium carbide arcs. 
Efficiency and size of light unit. 
‘The mercury are. 
Its ultra-violet light and production of ozone. 
‘Vacuum tube lighting. 
The incandescent lamp. 
Carbon filament. 
Chemistry of the methods of manufacture. 
Forming, baking, firing, coating and metallizing. 
Osmium filament. 
Tantalum filament. 
Tungsten filament. 


Only a few years ago anyone studying the chemistry proper of 
the sources of artificial illumination might well have been led to 
conclude that he could confine his efforts to a single element, i. e., 
carbon. This was owing to its general and peculiar applicability 
in all types of artificial lighting, no matter how widely they differed 
in their methods of employment of this interesting element. I even 


94 ILLUMINATING ENGINEERING 


think he might have been forgiven for assuming that in relation 
to light carbon occupied some such particular place among the ele- 
ments, as it does in the chemical relations of life. Carbon, of all 
the elements, is the basis of organic chemistry and the one funda- 
mental element without which organic substance and life itself are 
impossible. All artificial hght was at that time due to carbon 
heated to incandescence. The efficiency of the light sources de- 
pended on the efficiency of maintaining carbon at a high tempera- 
ture. In the various types of oil lamps which were in use several 
thousand years ago, the light is due to the incandescence of carbon. 
This carbon is a product of decomposition of the vapors of the oil. 
It can easily be deposited from the flame and be kept from burning 
by introducing a cooled surface into the flame. This service of 
the carbon is a double one in the case of oil and ordinary gas 
illumination. Here an element is needed which forms readily 
vaporizable compounds or gases, and compounds, too, which are 
decomposed by the moderate heat produced by the reaction of the 
compound with the air, and, finally, the element must itself be non- 
vaporizable at the temperature of the continuing reaction. In these 
respects carbon is apparently the only element which possesses the 
needed properties. It did not follow of necessity that this same 
element should be best suited for electric are lights and for incan- 
descent filaments, and yet for half a century it was the mainstay 
for both methods of illumination. Possibly it is this apparent 
selective fitness of carbon among the 77 elements that caused post- 
ponement of attempts at discovery of other methods of illumination. 

In an address of this kind on the chemistry of luminous sources 
(a subject selected to properly fit into a comprehensive scheme 
covering illuminating engineering), it seems best to spare special 
emphasis of selected kinds as much as possible, and to consider in 
something of a co-ordinating way the chemistry of all the prac- 
tical methods of lighting. 

In such a consideration one is soon impressed with the fact that 
the several different types of illumination differ relatively little in 
their net efficiency. The labor and material involved in the pro- 
duction of the light of a candle does not seem to differ much by 
whatever methods one employs to produce the light. A candle- 
power from a modern oil lamp, an alcohol lamp, from a gas 
lamp, or from an electric lamp is, speaking quite generally, a matter 
of about the same order of magnitude of cost. This would not 


THE CHEMISTRY OF LUMINOUS SouURCES 95 


be so remarkable if they were all nearly perfect illuminants, or if 
they were all of very high degree of energy efficiency—i. e., if they 
were all nearly perfect—but they are not. 

That they are nearly alike in cost is due to the fact that they 
are all so far removed from the perfect artificial illuminant that 
the large proportion of wasted energy practically determines the 
cost. ‘The kerosene oil lamp uses a few tenths of 1 per cent of the 
energy of the combustion of the oil in the production of visible 
light waves. The temperature at which the carbon is heated in this 
flame is so low that almost any other way of heating the carbon 
will give more light. In the case of the very intense acetylene 
flame we probably see the effect of much higher temperature of the 
carbon particles, as this is a hotter flame than that produced by 
common gas. It is known that the luminous radiation rises ex- 
ceedingly rapidly with rise of temperature at burning tempera- 
tures, so that the carbon does not have to be heated very much 
hotter in order to give off a very much greater light. Probably 
the range of temperature within which carbon is heated in the 
various kinds of lamps, excepting the arc and acetylene flame, lies 
below 1800° C. 

When ordinary illuminating gas is used, the maximum light is 
gained by a selected composition of the gas and construction of 
the burner. 

This is almost equal to saying that the gas is so mixed with the 
air which combines with it that none of the carbon produced by ~ 
decomposition of the gas is allowed to escape as soot, but is, on the 
other hand, kept heated without combustion within the flame as 
long and at as high a temperature as possible. If more air were 
introduced into the flame, less light would be produced, but a 
locally higher temperature. This is due to the increased rapidity 
of combustion of the carbon. This fact led to the introduction of 
other materials than carbon into the flame to be heated by the 
burning gas. Naturally, very little advance was made along this 
line until a scheme for making total and rapid combustion of the 
gas was developed. This was the work of Bunsen, who found 
that air mixed with the gas in suitable proportions brought about 
the effect of raising the temperature of the gas flame. In this appli- 
cation the carbon is immediately consumed and does not lend any 
luminosity to the flame. The industry waited at least a decade 
for some suitable substitute for the luminous carbon. It was the 


96 ILLUMINATING ENGINEERING 


exhaustive work of Dr. Auer von Welsbach which produced the 
mantles of metallic oxides which we know to-day. These, when 
heated to the high temperature produced by the combustion of 
mixed air and gas, give a much greater light for a given rate of 
gas supply than the previous method of use of the same gas. This 
increased light efficiency is also greatly augmented by the proper 
selection of the components of the mantle mixture. It would, at 
first thought, seem probable that any white mantle capable of with- 
standing the high temperature of the flame would give the same 
definite, constant quantity of light under the same conditions 
of heating gas flame. That this is not so is readily shown by a 
study of the efficiency of various oxide mixtures when used as 
mantle compounds. There are a number of metallic oxides which 
do not melt or vaporize at the temperature of the flame, but the 
most refractory is not the most satisfactory. Each mixture of 
oxides seems to have its own characteristic light-giving power, and 
to possess also some considerable selective power in producing color 
differences. 

This has led to an immense quantity of purely experimental 
research, in order to discover what particular compound or mixture 
would give the most efficient and satisfactory light. As an illustra- 
tion of this fact, it is worth noting that Welsbach discovered that 
pure thorium oxide, when used in a mantle, will not give a tenth 
of the light that will be produced under the same conditions by a 
mantle made of a mixture of 99 parts of thorium oxide and 1 part 
of cerium oxide. This very interesting phenomenon will doubtless 
be taken up by Mr. Whitaker, and is therefore only referred to 
at this point. An instructive article on this subject was published 
in the April, 1909, number of the Journal of Industrial and Engi- 
neering Chemistry. It is the one discovery which has apparently 
given the illuminating-gas industry the help it needed to keep in 
competition with methods of electric lighting. 

Just as no story of incandescent electric lighting can be properly 
started without at least a reference to the enormous contribution 
of Edison, so also any history of are lighting properly commences 
with Sir Humphry Davy. In 1809 he was experimenting with 
phenomena produced by a battery of 2000 primary cells, and pub- 
licly showed that a very luminous are was produced when the cur- 
rent passed across the gas between carbon points. While he may 
not have been the discoverer of the arc, he was one of the first to 


THE CHEMISTRY OF LuMINOUS SouURCES 97 


see a use for it. For a great many years thereafter no practical 
application was made of this discovery, because there had not been 
developed any satisfactory devices for generating the large amount 
of electrical energy consumed by even a small carbon are. In 1870 
the Gramme generator was devised. Carbon are lamps were oper- 
ated from this machine, in place of batteries. Some of the first 
attempts at practical use of these machines and lamps were made 
in connection with light-houses on the English and French coasts. 
Soon thereafter the Jablochkoff electric candle came into use. This 
is an arc lamp with parallel carbons. These were kept separated by 
a thin wall of clay, or a mixture of sand and glass, which gradually 
vaporized during the burning of the are. At one time several 
thousand of these were in use in Europe. At the Paris Exhibition, 
in 1878, the illumination produced by these candles, operated by 
Gramme machines, marked an epoch in lighting which the previous 
30 years of laboratory experiment with arcs had but dimly fore- 
shadowed. 

Somewhat later the simple carbon arc was commercially realized, 
and the clay part of Jablochkoff candles disappeared from the elec- 
tric lamp for a time. 

The phenomenon of this direct-current carbon arc is still quite 
far from being perfectly understood. From the chemical stand- 
point, the are presents two pure carbon pencils, each of which is 
slowly consumed. In the ordinary lamp the consumption of the 
positive, which is usually the upper electrode, is much more rapid 
than that of the lower or negative electrode. It was long evident 
that the wasting away of the carbon electrodes was largely due to 
simple combustion by the air, and many attempts were made to 
prevent this combustion, while retaining the characteristics of the 
carbon arc. This Jed to the discovery that the upper electrode is 
heated much hotter than the lower during the passage of the cur- 
rent, that carbon actually distills from this positive electrode, and 
when this carbon cannot burn it will deposit upon the cooler parts 
of the electrode. This property of building out mushroom growths 
on the electrodes when operated in vacuo or in inert gases seemed 
to stand in the way of economizing in such a lamp by practically 
separating the ordinary combustion of the electrodes from the 
proper electric-are phenomena. It was finally found, however, that 
by properly controlling the current and voltage, and by admitting 
only a very small quantity of air to the globe of a carbon arc lamp, 


98 ILLUMINATING ENGINEERING 


the combustion of the electrodes could be greatly reduced. This 
air rate, which is controlled by the openings in the supports of 
the inner globe of the enclosed arc lamp, so greatly reduces the 
burning of the electrodes that the life is increased ten-fold or more. 
This gives us, then, the two primary types of carbon are lamps, 
the open and enclosed. In the closed, as in the open, it is the 
positive electrode which wastes or burns away the more rapidly 
of the two; it is the hotter and is the source of most of the light 
from the are. In the pure carbon arc only a few per cent of the 
light is due to the flame or are proper. This are stream is far 
from dense, and most of the carbon in the space is already present 
as carbon monoxide. 

While it is out of place here to go very deeply into the con- 
ceptions of theories which have been formed to cover the action in 
the arc, it may not be amiss to point out that the simplest ideas 
are not applicable. For example, it is quite apparent that a 
motion of positively charged particles across the gap of the are 
does not account for all the phenomena. As will be seen more 
clearly later, the negative electrode, at least in most cases, is the 
one which determines the character of the arc, and a carbon are 
is still a carbon are when the positive electrode is some other con- 
ducting substance, while it is usually no longer a characteristic 
carbon are when the negative electrode is another substance. There 
is no simple quantitative relation known between the current car- 
ried in an are and the waste or loss at either electrode. In this 
respect the arc differs from the passage of current through a gap 
within a solution, for example. Attempts made to determine the 
minimum loss of electrode for a given are current have only led 
thus far to the conclusion that if any quantitative consumption of 
electrode takes place of necessity when an arc is passing, the quan- 
tity of material corresponding to a given current is at least a thou- 
sand times smaller than migrates when equal current passes through 
a solution or an electrolyte. Moreover, it seems that this motion 
within the arc is usually, if not always, made up of material from 
the negative electrode. This general subject has led to a great 
deal of quantitative work in which are electrodes of other mate- 
rials than carbon have been used. In: most cases, as with carbon, 
the results are affected by the simultaneous oxidation of the elec- 
trodes. Copper and iron electrodes, when used as are terminals, 
show such irregularities that it has been impossible to accurately 


THE CHEMISTRY OF LUMINOUS SOURCES 99 


determine values of loss at cathode or anode which might corre- 
spond in some way to the Faraday equivalents in electrolysis. Even 
when such arcs are operated in inert atmosphere or under water, 
one usually finds that the material of either electrode has passed 
in some irregular degree to the other electrode and deposited upon 
it. Such effects may be largely accredited to simple distillation. 
Some cases have, however, been found in which the processes of 
combustion may be fairly well separated from those of current 
action, and here again it seems proven that in an arc it is essential 
that material pass from the cathode into the arc space only, and 
that a consumption of the anode or positive electrode is always an 
accidental accompanying effect. ‘This will be referred to later. 

‘We have thus far considered only the chemistry of the pure 
carbon arc. Modification of this are of interest to illuminating 
engineers have been many. It seems necessary to refer briefly to 
a few of them before considering other arcs. The direct-current 
carbon arc owes its efficiency to the highly heated crater or arc 
terminal on the positive carbon. When an alternating-current 
carbon lamp was measured, it was found that not quite so great 
efficiency was possible, though by the alternating position of the 
crater with each change in current direction, the distribution of 
the light is somewhat improved. 

Many inventors have attempted to increase the light from a 
given are energy by introduction of suitable chemical compounds 
into the arc. Some of these have led to successful commercial 
lamps. If a small piece of a very refractory material, such as 
zirconia, be brought into the carbon arc, it is heated to a tempera- 
ture at which it is very luminous. This is quite like the use of a 
rod of lime in the Drummond gas lamp. The difficulties in the 
way of stability, of mechanism, ignition and control, may account 
for the failure to develop this device in its simplest form. 

A small zirconia rod placed between the two carbon electrodes 
(when arranged as ordinarily, one above the other), although 
patented as an arc lamp, has not been commercially developed. A 
modification of this scheme, whereby a special form of Welsbach 
mantle is placed about the carbon arc to be heated by the arc, has 
also not advanced very far. A considerable difficulty in such schemes 
lies in the fact that the hot path of the are stream is usually of 
very small cross-section, and in lamps of moderate energy consump- 
tion is not easily confined to a limited position, so that it is not 


100 ILLUMINATING ENGINEERING 


easy to keep interposed material heated to incandescence by this 
means. 

Countless schemes for continuously introducing powders or va- 
pors into the are have also been tried. It was found many years 
ago that the addition of such salts as carbonate of soda to carbon- 
are electrodes gave added luminosity to the are, reduced the volt- 
age across the arc and also permitted the arc to be lengthened with- 
out extinguishing it. Very small quantities of such salts are 
effective. This general knowledge did not produce the modern 
flame arcs at once, as the effect of such salts as were used a quarter 
of a century ago was probably not greatly marked or appreciated. 
About 10 years ago inventions involving this principle became quite 
common. Perhaps best known among them are those of Blondel 
in France and Bremer in Germany. They and others made use 
of very considerable proportions of salts added to the carbon during 
the manufacture of the electrode. Usually 10 per cent or more of 
mineral substance was added, and many different salts were pro- 
posed. Most successful seem to be the fluorides and chlorides of 
calcium and magnesium. Some inventors found they were able 
to construct an operative electrode by using a homogeneous rod of 
carbon and the salts. Others preferred to confine the salt to a core 
inside one or both electrodes. In most cases this core also contained 
some special form of carbon, and in some cases there were two 
concentric cylinders of various composition about the central core. 
It has been quite common to use carbon electrodes with a core of 
soft carbon, as the arc by this means is kept centered on the elec- 
trode. The present so-called carbon flame arcs, which are usually 
characterized by great luminosity, with predominance of reddish- 
yellow color, are made in the above way. ‘The electrodes usually 
contain so much mineral matter that they cannot be used in en-. 
closed lamps of the ordinary types. The mineral matter, after 
passing into the arc, must be carried from the lamp by a good 
draught, otherwise it will deposit on the globe and soon greatly 
reduce the luminosity of the lamp. The necessary draught involves 
also the rapid consumption of the electrodes, so that such lamps 
usually have to be trimmed or supplied with new electrodes daily. 
The presence of the salts insures low voltage for the lamp, so that 
they are usually burned two in series on the 110-volt circuit. 

The most useful future application of chemistry to this type of 
flame are lamp will doubtless be along the hnes of producing as 


THe CHEMISTRY OF LUMINOUS SOURCES 101 


great an efficiency in white light as is now produced in the reddish 
tint. ‘Taken as an electric-light source alone, these reddish-flame 
arcs are the most efficient of any of the commercial lamps. I attach 
a table of efficiencies of various kinds of electric lamps for com- 
parison. Such a table, taken alone, may be very misleading. No 
indication of color, convenience, size of unit, and other practical 
considerations, appear in such a table. 


W.P.C. 
Carbon (open arc) ..... DC; 10 A. 43 V. 1.43 (spherical) 
ce (enclosed) ..... Ww AKGS 5 (CA. 80 V. 2.27 ‘ 

i. (enclosed) ..... A.C. (Doe. 80 V. 2.47 a 

Sw ervor tame arc ...... Dio: TOP A. 45 V. 42 "s 
Maenetite are bee oo... DiC: 4 A, 80 V. 1.25 cs 
BLOUAS GUN pac whe tee ee ara DAC. 5 110 V. 1.7 (horizontal) 
Metallized carbon ...... 2D 110 V. 2.6 i 
Se 0 es On a 5 110 V. Pal vs 
RS Rc oO es D.C, 35D 6 . 

3 (pressure) ..... D.C. 3 i: 
WO RCS re gos p sials 6 3 AO. 1.6 a 
PE es ye ew vs ss Both 20 Le x 
Cee Be ee. mi 45 art "s 
eterno ETT Ls. 243 1.25 * 


It is particularly in the arcs that the chemical nature of the 
electrodes plays a determining part. When a ‘simple carbon arc 
is considered, the quality of the carbon is of the greatest im- 
portance. Pure graphite is not acceptable, but a hard, dense carbon, 
quite low in ash and of very fine physical structure, is most satis- 
factory. For many years these were imported from Germany, and 
they still are to some extent. 

In the introduction of new substances to the carbon arc there 
are many chemical and physical properties which unite to determine 
the value of the added substance. The salts of many elements add 
more or less intense colors to the arc, in accord with the spectrum 
lines of the particular element. This effect is greatly influenced by 
the degree of volatility of the salt and by the nature of the other 
elements or compounds vaporizing at the same time. Calcium 
oxide does not greatly affect the luminosity of the carbon-are stream, 
while calcium fluoride does. 

During the past 10 years some advances have been made in the 
practical use of other arcs than carbon. The best known are the 
magnetite and the mercury arcs. 


102 ILLUMINATING ENGINEERING 


The magnetite differs chemically from the carbon in being much 
less combustible, as it burns only in changing from Fe,0, to Fe,O., 
in giving non-volatile oxides and in giving to the are flame, to a 
high degree of intensity, the characteristic colors of the iron spec- 
trum. The iron spectrum is one of those metal spectra which, 
while made up of defined lines, contain such a great number of 
them (over 2000 have been mapped) that the effect is practically 
that of a continuous spectrum. In the magnetite are practically 
all of the light is due to the are or flame. The luminous positive 
of the carbon arc is in this lamp replaced by a large block of copper 
or other metal, which does not contribute to the consumption in the 
arc, so that this lamp is an are lamp with only a single consuming 
electrode. The quality of the arc is greatly influenced by the 
quality of the magnetite electrode. It might seem probable at first 
that iron itself would be preferable to magnetite, but long series of 
experiments seemed to show that a compound and rather complex 
mixture, containing large proportions of pure magnetite, gave the 
best results. Such arcs must burn steadily and the electrode must 
contain a small amount of relatively volatile matter, such as the 
common salts of potash or soda. For a given current the rate of 
waste of the electrode can be very materially altered by the addi- 
tion of otherwise inactive materials, such as alumina and chromium 
oxide, without any considerable reduction in the light produced. 
This effect is probably due to the reduction of vapor pressure of 
the iron oxide in the molten top of the electrode. This corre- 
sponds to vapor-pressure reduction in case of simple solutions. 
Finally, it was found that the intensity of the arc is greatly in- 
creased by the addition of another element which has its own rich 
spectrum, such as titanium. So that the magnetite arc is really 
the are spectra of iron and titanium superposed. Such strictly are 
flames have one advantage over carbon arcs, in that they can operate 
economically in small units. The efficiency of the carbon arc is 
greater the larger the unit within a wide range, but units below 
500 watts begin to be relatively inefficient. On the other hand, the 
efficiency of the strictly luminous arcs is maintained high as low as 
250 or 300 watts. This, to the illuminating engineer, means that 
he has greater elasticity in the distribution of his lighting energy. 

The mercury arc may be said to differ but little from the other 
ares. It is greatly lengthened by being confined to a glass tube, 
and thus any combustion or loss of material is obviated. Its color 


THE CHEMISTRY OF LumMINOUS SouRCES 103 


and light are determined as in the case of other arcs, by the nature 
of its cathode electrode. ‘The anode, as in the other arcs, may be 
made of almost any conductirig material. The vapors which are 
produced at the cathode condense to liquid state and return by 
gravity to the cathode. If the chemical elements had more fluid 
members among those of highly luminous spectra, the principle of 
the enclosed mercury lamp would probably quickly yield more new 
and useful lighting methods. The light of the mercury lamp, when 
broken down by the prism, is seen to be composed of only a few 
widely separated lines. Among them is no red. For this reason 
red articles appear black under this light, and, for this reason, 
many other colors fail to appear natural under the mercury arc. 

There are two interesting facts concerning the mercury are which 
may well ultimately be utilized in a practical manner. The are 
is very rich in ultra-violet light. This is not particularly noticeable 
when the arc is surrounded by glass, but when pure quartz is sub- 
stituted for the glass the ultra-violet light penetrates into the sur- 
rounding air. This produces ozone in a very marked manner, and 
this unfiltered light has a very serious and injurious effect on the 
eyes. It is highly probable that this modified mercury lamp is to 
be the most readily applicable form of ultra-violet light for thera- 
peutic purposes. Secondly, it has been discovered that when the 
are is operated under two or three atmospheres of mercury pressure 
the efficiency is high and the color more nearly approaches day- 
light. Glass tubes will not withstand the temperature of the arc 
at this pressure, but quartz will. Such. quartz mercury lamps are 
being made and sold abroad at the present time. 

Any considerable practical improvement in the color of the mer- 
cury arc has not been made by the amalgamation of other elements 
with the mercury. An element like copper or iron fails to vaporize 
from the cathode of the mercury are. Some of the alkali metals 
somewhat alter the light, but most of them also attack the glass 
of the lamp. It is worthy of note that some fluorescent dies, rhoda- 
mine, for example, are capable of absorbing the green and blue 
spectral lines and returning in their place some considerable red, 
but this has not proven an efficient process. 

The luminosity of gases and vapors has always. seemed a very 
promising field of artificial illumination. In the case of heated 
solids, the laws of radiation, convection and conduction are well 
enough known, so that a field in which less is known is apt to seem — 


104 ILLUMINATING ENGINEERING 


promising. The Geissler or Pliicker tubes, in which attenuated 
gases are rendered luminous by relatively high voltage and low- 
current discharge, are well known to all. It seems very probable 
that future developments of importance will be made, and already, 
in the McFarlane-Moore System, very considerable advances have 
been made. Here the chemical composition of the gases and their 
pressure are the determining factors of the color and efficiency. 
A peculiar phenomenon in these lamps is the apparent consumption 
of the gas or air in the tubes. Gradually, in such apparatus, the 
gas disappears, as though driven into or combined with the glass. 
For this reason the inventor of this system has devised an automatic _ 
inlet valve which operates to let gas into the lamp when the vacuum 
rises to a certain degree. This seems to be a similar effect to the 
well-known “ hardening ” of X-ray bulbs from continued use, which 
is an improvement in vacuum, and is also noted in the case of the 
vacuum of an ordinary incandescent lamp. 

Without wishing to go deeply into the history of the incandescent 
lamp, it is necessary to point a moment to the work of Mr. Edison. 
The fact that electric current flowing through a conductor could 
heat it to incandescence had long been known. ‘That carbon in 
filament form, when preserved from combustion by a vacuum, would 
make a lamp was clear. J. W. Starr had patented such a lamp in 
1845, and Swan, in England, had exhibited one in 1879. But 
between this point and a satisfactory incandescent lamp was a great 
eulf, which needed the untiring energies of such an inventor as 
Mr. Edison to help bridge. , A piece of carbonized thread, confined 
in such a vacuum as was known when he undertook the work, did — 
not constitute a practical lamp at all. In the poor vacuum produced 
by methods used in those days, even a good filament of the present 
time would have produced but a very imperfect lamp. The simpler 
methods of producing carbon filaments are capable of yielding only 
very imperfect lamp filaments. There are few artificial products 
which excel the filament in the divergence between apparent sim- 
plicity and actual complexity. ; 

The choice of elements for incandescent-lamp filaments may be 
said to be more nearly a physical than a chemical problem, but in 
the manufacture of all of them chemistry plays a dominant role. 
The best carbon filaments now in use may be described as con- 
sisting of a core of pure carbon, not graphite, covered with a coat 
or shell of pure graphite, which has been so changed by an electric- 


THE CHEMISTRY OF LUMINOUS SOURCES 105 


furnace treatment, under atmospheric pressure, that it has a posi- 
tive-resistance temperature coefficient instead of a negative one. 
This graphite coating, to which the name metallized graphite has 
been given, has the appearance of having been melted or sintered 
together, and thus differs from all other graphite. 

The chemical and physical processes by which these carbon fila- 
ments are produced are as follows: 

High-grade cotton is dissolved in a strong solution of zinc chloride, 
which is then squirted through a small hole into dilute alcohol. The 
alcohol coagulates the viscous solution of cellulose so that a trans- 
parent thread is the product, and by washing this in running water 
the zine chloride is removed. 

Another equally satisfactory method for reaching the same end 
is to squirt a thick solution of nitro-cellulose, dissolved in acetic 
acid, into a container holding water. Washing with ammonia 
sulphide and water changes the nitro-cellulose into non-explosive 
hydro-cellulose. This product is then dried in the air while stretched 
on drums. It is then cut to desired lengths, formed into the nec- 
essary loops on brass frames, and finally packed in graphite boxes 
in a packing material such as baked peat, and very gradually heated 
until carbonization takes place. In this process the carbonized 
filaments are heated to as high a temperature as can be obtained 
by gas or oil-heated muffles. 

The product at this point is dense, hard carbon, A even 
under the microscope, is far from having the appearance of charcoal, 
and seems almost free of pores. The carbon filament in this form 
would make a very inferior lamp. The color or quality of its sur- 
face, and probably the volatility of its material, is not nearly so fa- 
vorable to lamp making as the corresponding properties of graphite. 
At any definite operating energy the amount of light produced 
by a gray-graphite surface is greater than that produced by a 
black-carbon surface, so that the carbon filaments are graphite- 
coated. This is done by heating them by the current in an atmos- 
phere of hydro-carbon, such as benzine, at low pressure. The quality 
and thickness of the coat may be controlled by the duration and 
temperature of the treatment. Until a few years ago the greater 
part of all carbon filaments were made in this way. It was then 
found that the effect of subjecting the graphite-coated filaments to 
temperatures above 3000° C. for a few minutes changed the graphite 
very materially in its properties. ‘Those which are of interest to 


106 ILLUMINATING ENGINEERING 


us now are the resistance, its temperature coefficient and the sta- 
bility at operating lamp temperatures. Briefly, the resistance of 
the graphite coat is reduced to about 20 per cent of its original re- 
sistance. Its temperature coefficient is reversed and its lasting 
powers in the lamp increased nearly three-fold. 

This point seems a proper one at which to mention the standard 
of use for incandescent lamps as determined by practical conditions. 
Burning at a low efficiency, an incandescent lamp has practically 
an indefinite life. At 3 watts per candle-power it may have 1200 
hours’ life and at 2.5 about 500 hours to 80 per cent of its original 
candle-power. It has been found by use that about 500 hours’ life 
for a carbon lamp is most practical, this 500 hours being the length 
of time the lamp remains above 80 per cent of its starting candle- 
power. ‘The metallized filament lamps, therefore, instead of being 
burned at the former efficiency of 3.1 watts per candle, are made 
to burn at about 24 .w.p.c., at which they have about 500 hours’ 
life. Evidently the higher the cost of the lamp the more stress has 
to be laid upon long life, while with very cheap lamps there is an 
advantage gained by burning them at unusually high efficiency and 
replacing them at the end of much less than 500 hours. 

The history of the development of the various metallic filament 
lamps is particularly interesting from the chemical standpoint. In 
the early days of incandescent lighting Mr. Edison and others rec- 
ognized the peculiar value of metallic filaments because of their 
flexibility and electrical conductivity. At that time platinum and 
iridium were the metals which offered most promise. ‘They were 
the metals of highest melting point, so far as then known. It was 
soon apparent that these metals could not be run at high enough 
temperature to make a practical lamp, though they were very nearly 
suitable. Mr. Edison then carried out a great number of experi- 
ments in an attempt to raise the melting point of the platinum. 
The effect of the occluded gases was carefully studied, but a com- 
mercial lamp did not result. For over a quarter of a century there- 
after, it remained unknown that at least six or seven of the then 
known metals had higher melting points than platinum. The en- 
tering wedge into this field was driven by Dr. Auer von Welsbach, 
who had acquired a personal and almost exclusive knowledge of a 
large group of more or less rare chemical elements in connection 
with his extensive researches, which were crowned by his gas-mantle 
inventions. At this time probably none of the metals which melt. 


THE CHEMISTRY OF LUMINOUS SouRCES 107 


higher than platinum had ever been produced in any other form 
than that of a fine black powder. Osmium was the first of a trio of 
metals to become a nearly practical filament. It occurs in nature 
in metallic state, usually alloyed with iridium, platinum, rhodium 
and ruthenium. It is found only as very small grains or plates, 
and nowhere in any considerable quantity. By mixing powdered 
metallic osmium with a suitable starch or sugar binder, Welsbach 
squirted a thread which, after drying and baking, could be freed 
of carbon by heating in a mixed atmosphere of hydrogen and water- 
vapor. The resulting metallic filament was quite soft when hot, 
but was well suited for incandescent lamps, as it withstood tem- 
peratures necessary to produce a lamp burning satisfactorily at 
about 1144 watts per candle-power. The world’s known supply of 
osmium is very small, and to conserve this supply the lamps were 
usually rented instead of being sold. 

In 1901 Dr. Werner von Bolton announced the discovery of 
ductile tantalum. Operating in an incandescent lamp, it could be 
burned at about 1.7 watts per candle-power for a thousand or more 
hours. The metals tantalum and niobium are a pair usually occur- 
ring together and formerly quite difficult of separation. They 
occur in small quantities in Connecticut, in the Black Hills of 
Dakota, in Sweden and in Australia, the mineral being usually 
tantalite (a compound of the oxides of tantalum and iron, with 
or without manganese or tin) or some combination of tantalum and 
niobium oxides with iron, etc., as columbite, samarskite, fergu- 
sonite, etc. It was necessary’ first to perfect methods of preparing 
the pure metals, and of these the tantalum was found to have the 
higher melting point. It is about 3100°, while that of niobium is 
about 2900°, or still well above platinum. 

Until this investigation it had apparently been known only as 
powder. This powder was melted together into large buttons in 
an electric are and then drawn to wire in the usual manner through 
diamond dies. | 

Probably most, if not all, of the tungsten filaments in the lamps 
on the market are made by some method of squirting through a 
die tungsten powder mixed with a binding agent. The metal, in 
finely divided state, is usually obtained by the reduction of tungstic 
oxide at a red heat by hydrogen. This oxide is in turn obtained 
from the minerals Wolframite, which is a tungstate of iron or iron 
and manganese, and Scheelite, a tungstate of calcium. Several 


108 ILLUMINATING ENGINEERING 


thousand tons of ore, averaging over 50 per cent tungstic oxide, 
are mined annually, largely for use in high-speed tool steel. 

Some of the successful processes for making the filaments are as 
follows: 

The powdered metal is mixed with a proper carbonaceous binder, 
then formed into threads by being forced through a suitable die, 
dried and baked at about red heat. They are then heated by pas- 
sage of current through them in a suitable atmosphere of hydrogen 
or mixture of hydrogen and nitrogen. By this treatment a shrink- 
age of the filament takes place, it becomes dense and metallic in 
appearance, and at the same time the carbon present is removed.. 
The product is, therefore, pure tungsten. 

Similarly, a metallic binding agent may be used. The finely 
divided metal in one such process is mixed with a cadmium-bismuth 
amalgam and the resulting mixture is pressed through a die. A 
thread not unlike a fine, lead fuse wire is the result. On heating 
this in in vacuo all metals but the tungsten are vaporized, and at 
the final temperature this is also sintered together into a compact 
filament. 

In the case of tantalum, nature seems to supply just about enough 
of the ore to satisfy the demand, and probably this element would 
have been a more successful competitor in the incandescent-lamp 
field if it only had to contend against carbon and osmium. It was 
more efficient than the former and much more plentiful than the 
latter. It is interesting to recognize the fact that the most recent 
successful metal filament, tungsten, occurs in nature in abundance. 
It was discovered by Scheele in 1781. For over 200 years it was. 
known in the pure state only as an infusible gray and heavy metallic 
powder. Its melting point, as determined by Pirani, is 3350°, and 
is the highest melting point of which we have measurement. The 
only measurement of higher temperature on the earth is that of the 
carbon-are crater, said to be about 3500° C., by Burgess and ‘Waid- 
ner. In all types of incandescent lamps there lies a promise that 
continued study will give continued advance in the art. This is 
sought usually as higher efficiency. A carbon lamp will burn a few 
moments at an efficiency 10 times as great as its normal value. In 
other words, from the materials at hand, this increase in efficiency 
is possible for a short time. It seems, therefore, not impossible. 
that this limiting time feature may be better controlled when 
better understood. 


Jia 
ELECTRIC ILLUMINANTS 


By CHARLES PROTEUS STEINMETZ 


CONTENTS 


GENERAL 

. The different forms of radiators and different kinds of radiation. 
Classification of electric illuminants. 

. Importance of the volt-ampere characteristic and the resistance- 
temperature characteristic of the conductor used in electric 
illuminants. Discussion of the multiple or constant potential, 
and the series or constant-current electric distribution system. 


SoLip CONDUCTORS 


. Volt-ampere and resistance-temperature characteristic of incandes- 
cent lamp filaments. Positive and negative temperature coeffi- 


: d Se ; : 
cients, — >0. Stability of operation on constant potential and 


on constant current circuits. [Fig. 1: Volt-ampere characteristics 
of incandescent lamp filaments. Fig. 2:  Resistance-character- 
istics of incandescent lamp filaments. | 

. Volt-ampere characteristic of pyroelectrolytic conductors. The 
Nernst lamp glower as pyroelectrolyte. The instability range, 


d 
aH <0, of pyroelectrolytes on constant potential supply, and the 


necessity of steadying resistance or reactance. The Nernst lamp. 
[Fig. 3: Volt-ampere characteristic of low resistance pyroelectro- 
lyte. ] 

. The light radiation of solid conductors, as incandescent lamps and 
the Nernst lamp glower. Black-body, gray-body and colored- 
body radiation. Effect on the efficiency of the incandescent lamp 
filament and the Nernst lamp glower. Limitation of efficiency. 

. Relation of refractoriness and vapor tension or disintegration, to 
the possible efficiency of the incandescent lamp. Comparison of 
the carbon filament with the metal filaments. 

. The production of the carbon filament lamp. Base carbon and 
treated carbon, and their stability. 

. Metallized carbon, its resistance and temperature coefficient, and the 
gem lamp. 

. Metal-filament incandescent iamps. Osmium lamp, tantalum lamp, 
tungsten lamp. Their efficiencies. 


110 ILLUMINATING ENGINEERING 


10. 
Be 
12. 
13. 


14. 


15. 


16. 


17. 


18. 


a9: 


20. 


21. 


22. 


The manufacture of the tungsten lamp. 

Thinness and length of metal filaments. Fragility. 

Efficiencies of the different incandescent lamps. Conventional rat- 
ing in horizontal candle-power. Relation of efficiency to useful 
life. 

Relation of the efficiency of the incandescent lamp to the size of the 
unit, or the power consumption. Limitation by supply voltage at 
small units, by size of the lamp globe at large units. Wide range 
of units with fairly uniform efficiency. 

Inferiority of the incandescent lamp in efficiency, to the flame arc 
and luminous arc.. Superiority in small units. Main field of 
application of incandescent lamps and Nernst lamps in small 
units, where no other electric illuminant exists. 


GASEOUS CONDUCTORS . 


Difference between disruptive or Geissler-tube conduction, and con- 
tinuous or are conduction. 


' GEISSLER-TUBE CONDUCTION 


Electric characteristics of Geissler-tube conduction: total voltage, 
terminal drop and stream voltage as function of gas pressure. 
[Fig. 4: Volt-pressure characteristic of Geissler tube with air as 
conductor. Fig. 5: Volt-pressure characteristic of the Geissler 
tube with mercury vapor as conductor.] 

Performance. efficiency and color of light. The Moore tube 


Arc CONDUCTION 


Nature of the are conductor. The are as unidirectional conductor. 
Rectification by the are. The alternating current arc. Constant- 
pressure and varying-pressure arcs. 


d 
Volt-ampere and volt-length characteristics of the arc: =: ee th 


[Fig. 6: Volt-ampere characteristic of magnetite arc of .5, 1.5 and 
2.5 cm. length. Fig. 7: Volt-length characteristic of magnetite 
are at 2, 4, 8 and 16 amperes. ] 

Dependence of the arc voltage on two independent variables, current 
and are length. Instablity of the are on constant voltage supply. 
Necessity of steadying resistance or reactance. The stability 
curve of the are. [Fig. 8: Stability curve of the 1.5 cm. magne- 
tite arc.] 

Instability of parallel operation of arcs without steadying resis- 
tances. Instability due to non-inductive resistance shunt. Ex- 
tinction by shunted capacity. The arc as interrupter. The 
singing arc. 

Stream voltage and terminal drop of the arc. Heating of the termi- 
nals by the terminal drop. The carbon arc as incandescent radi- 
ator. Relation between the efficiency of the carbon arc, and the 
size and the life of the terminals. 


23. 
24, 


25. 


26. 


27. 


28. 


29. 


30. 


31. 


32. 


33. 


ELEctTRIC ILLUMINANTS Ttt 


The open carbon arc or short burning are lamp. The enclosed 
carbon arc or long burning lamp. Its inferiority in efficiency. 
Uneconomical operation of. continuous-current series arc circuits. 

The series alternating enclosed arc lamp. Its very low efficiency. 

Replacement of the enclosed alternating carbon are by the magnetite 
arc lamp in street lighting, by the intensified are or the tungsten 
incandescent lamp in indoor lighting. The intensified arc lamp. 

The luminous are and the flame are. Their characteristic differ- 
ences, advantages and disadvantages. The magnetite arc. 

The flame carbon arc. Relation between size of electrodes and effi- 
ciency. The short-burning and the long-burning flame carbon 
are. The yellow color of the flame carbon are. Titanium, calcium 
and mercury as the three most efficient arc stream radiators. 

The mechanism of the arc lamp: starting device, feeding device, 
steadying device, shunt protective device, damping devices. 
Series lamp, shunt lamp, differential lamp. 

The effective resistance of the arc. Relation between arc length and 
efficiency. The short carbon arc and the long luminous and flame 
arcs. 

Regulation of are lamp for constant light flux. The floating system 
of control of the carbon arc and its advantages. Fixed are length 
required by the luminous arc. Its difficulties in constant potential 
lamps. The compromise control of the flame carbon lamp. 

Classification of arc lamps; the most important forms of arc lamps: 

The open carbon are on 9.6 amperes series direct current 
circuits. 

The enclosed carbon arc, for multiple and series circuits, on 
alternating and on direct current. ; 

The intensified carbon arc, on alternating and on direct cur- 
rent circuits. 

The vellow flame carbon arc, on alternating and on direct 
current circuits. 

The magnetite arc. 

The mercury arc. 

Increase of the efficiency of the arc with increasing size of the 
light unit. Relation between the efficiency of the arc lamp and 
the current, arc length and power, at constant arc length, con- 
stant current and constant power. The condition of maximum 
efficiency. [Fig. 9: Efficiency and power consumption of the 4- 
ampere magnetite arc for different arc lengths. Fig. 10: Effi- 
ciency and power of the .7-inch magnetite are for different cur- 
rents. Fig. 11: Efficiency, arc length and voltage of the 300-watt_ 
and the 500-watt magnetite arc, for different currents. Fig. 12: 
Relation between voltage, current, arc length and efficiency of 
the magnetite arc, under the condition of maximum efficiency, 
for various powers. ] 

Comparison of the arc lamp and the incandescent lamp. 


113 ILLUMINATING ENGINEERING 


Vacuum ARCS 


34. The low-pressure mercury arc in the glass tube. The high-pressure 
mercury arc in the quartz tube. Their characteristics. 


SENSITIVITY TO VARIATIONS OF THE ELECTRIC POWER SUPPLY 
35. Comparison of various forms of incandescent lamps and arc lamps . 


regarding their sensitivity to variations of the electric power 
supply. 


GENERAL 


1. The Different Forms of Radiators and Different Kinds of Radi- 
ation. Classification of Electric Illuminants 


In the production of light from electric power, solids, liquids or 
gases (the latter including vapors) may be used as conductors of 
electric power, and the radiation may be due to incandescence of 
the radiator, that is, temperature radiation (black-body, gray-body 
or colored-body radiation), or it may be the result of a more or 
less direct conversion of the electric power into radiation, as 
luminescence. | 

Solids as conductors of electric power are used in the various 
forms of incandescent lamps: the different types of carbon-filament 
lamps and the metal-filament lamps, as the osmium lamp, the 
tantalum lamp and the tungsten lamp, and also in the Nernst 
lamp. liquids are not used as conductors, due to their difficulty 
of application, but gases and vapors are extensively used in the 
various forms of arc lamps, as the open and the enclosed carbon 
arcs, the flame arcs and the luminous arcs, which latter include 
the vacuum arcs, and in the Geissler tube as illuminant (Moore 
light). In the former, the arc lamps, the vapors of the electrode 
material are used; in the latter, the Moore light, the gas which fills 
the space between the electrodes. 

In all solid conductors, and also in the plain-carbon arc lamp, 
the light production is due to temperature radiation or incandes- 
cence, either black-body or gray-body radiation, or colored-body 
radiation. In the flame arcs, luminous ares (including vacuum 
ares) and Geissler tubes luminescence plays an essential part in the 
light production. 


ELECTRIC ILLUMINANTS 113 


2. Importance of the Volt-Ampere Characteristic and the Resist- 
ance-Temperature Characteristic of the Conductor Used in 
EHlectric Illuminants. Discussion of the Multiple or Con- 
stant Potential, and the Series or Constant-Current Electric 
Distribution Systems 


Since in electric illuminants the light is given by electric con- 
duction, the properties of the electric conductor, which is used in 
the illuminant, are of the foremost and fundamental importance, 
that is, the relation of current and voltage to each other, or the 
so-called “-volt-ampere characteristic” of the conductor; and the 
relation of the ratio of volts and amperes, that is, the effective 
resistance, to the temperature, that is, the “ resistance character- 
istic ” of the conductor. This is obvious, since the illuminant must 
be capable of use in the existing electric-power distribution systems. 

Electric power is distributed in two different forms: by the con- 
stant-potential or multiple-distribution system, that is, at the con- 
stant voltage of 110 or 220 volts,* or by the constant-current or 
series system. 

In the constant-potential system all apparatus are connected in 
parallel between the same supply mains, and thereby receive the 
same voltage, but each takes a different part of the supply current. 
All the illuminants must therefore be designed to operate at the 
same constant-terminal voltage of 110 or 220, and within such 
variations of this voltage as may be met in a constant-potential 
distribution system, which varies from 1 per cent to 5 per cent or 
more, depending on the character of the system. The different 
illuminants, however, may be designed for different currents. The 
multiple system has the advantage of permitting practically un- 
limited extension: with increase of the number of illuminants, the 
current in the supply feeders and mains increases, and larger con- 
ductors become necessary, but the voltage remains the same. When 
the number of illuminants becomes so large that the size of supply 
conductors becomes uneconomical, more sources of supply become 
necessary. Since, however, these sources of supply are usually sec- 


* 110 volts here means any constant voltage between about 105 and 
125, and 220 volts twice this value: not the same voltage is used in dif- 
ferent distributing systems, but slightly different voltages, for the 
purpose of making the economical production of exactly rated incandes- 
cent lamps possible. (See ‘‘ General Lectures on Electrical Engineer- 
ing,” by the author, p. 12.) 


114 ILLUMINATING ENGINEERING 


ondary stations, that is, transformers or converters receiving their 
power from a primary generating system at high voltage, this intro- 
duces no serious limitation. The constant-potential system of dis- 
tribution therefore is now generally used, with the exception of 
those few cases, where it is not economical: at the low voltage of 
110 or 220 volts, the distance to which electric power can be sent 
is rather limited. When numerous illuminants are scattered over 
a wide area this difficulty is met by secondary stations, as trans- 
formers, as stated above. If, however, individual illuminants are 
scattered over a wide area, as in street lighting, the individual 
illuminants cannot be reached from one 110- or 220-volt feeding 
point, while the installation of a transformer at every lamp is 
uneconomical, and in this case the constant-potential system becomes 
uneconomical and the constant-current system is used. For street 
lighting the series system is therefore universally employed, with 
the exception of those few places in large cities where the street 
lamps can be reached by a multiple system installed for general 
distribution. 

In the constant-current or series system all apparatus are con- 
nected in series with each other, and thereby receive the same 
current, and the voltages consumed by the different illuminants 
add. The illuminants therefore are designed for the same current, 
but may consume different voltages. Since the voltage of a dis- 
tribution circuit cannot be indefinitely increased without involving 
difficulties with insulation and danger to life and fire risks, the 
number of apparatus which can be connected into one series circuit 
is rather limited; a series circuit is a very small unit of electric 
power, from our present point of view, and as economy requires the 
use of the largest possible units series circuits are used only in 
those cases where they are economically necessary, that is, for 
street lighting. It was, however, with series arc circuits that electric 
lighting started in the early days. 

Series circuits are usually operated at 4, 5, 6.6 or 7.5 amperes, 
some of the old open carbon are circuits at 9.6 amperes, and with 
voltages ranging usually from 4000 to 6000. 

Not all conductors, and therefore not all illuminants, can be 
connected promiscuously into multiple circuits or into series cir- 
cuits, even if designed for the proper voltage respectively current, 
and the study of the electric characteristics of the conductors which 
are used in illuminants is therefore of importance for their design 
and operation. 


ELEctric ILLUMINANTS 115 


SOLID CONDUCTORS 


8. Volt-Ampere and Resistance-Temperature Characteristic of In- 
candescent Lamp Filaments. Positive and Negative Tem- 
perature Coefficients, e >0. Stability of Operation on 
Constant-Potential and on Constant-Current Circucts 


The conductors of incandescent lamps are ohmic resistances, 
that is, conductors in which the resistance does not directly de- 
pend on current or voltage, but is constant at constant tempera- 
ture, and if it varies with a change of temperature, in case of a 
negative temperature coefficient, that is, a decrease of resistance 
with increase of temperature, the decrease of resistance with in- 
crease of temperature is less than the increase of current required 
to cause the increase of temperature. That is, such conductors are 
characterized by the relation: 


de 
di. 

In other words, an increase of current always causes an increase 
of terminal voltage. If the resistance were perfectly constant, 
that is, the temperature coefficient zero, the voltage would be pro- 
portional to the current, and the volt-ampere characteristic given 
by a straight line going through the origin, I in Fig. 1, and the 
resistance characteristic given by a horizontal straight line, I in 
Fig. 2. No conductor exists which has zero temperature coefficient 
over more than a limited range of temperature. 

If the temperature coefficient is positive the resistance increases 
with increase of temperature, and the voltage thus increases more 
than proportional to the current; that is, an increase of current 1 
causes an increase of temperature and thereby of resistance r, and 
thus an increase of the voltage e=ir, which is greater than pro- 
portional to i, as shown in curves II to IV in Figs. 1 and 2. 
Inversely, if the temperature coefficient is negative the resistance 
decreases with increase of current, and therefore of temperature ; 
but the voltage still increases with increase of current, though less 
than proportional to the current, as shown in curves V and VI 
in Figs. 1 and 2. 

As illustrations are shown in Fig. 1 the volt-ampere character- 
istic, and in Fig. 2 the resistance characteristic of the conductors 
or filaments of various types of incandescent lamps. In Fig. 1 


0; 


116 ILLUMINATING ENGINEERING 


the co-ordinates have been chosen so as to start all curves at the 
slope of 45° at the origin. In Fig. 2 the co-ordinates have been 
chosen so as to give 10 at the operating point of the lamp. In 


Ra Gia aa f° 
psa | + fda ia) Solos VA 
2h 28 ARORA 7 a 
eg Tee aR Ee 7 
shou uon oy So | yaad loa 


CO 
i ch A il 





Fig. 1.—Volt-Ampere Characteristics of Incandescent Lamp Filaments. 


Fig. 2 as abscissae have been used \w, which with a black-body 
radiator would be proportional to the absolute temperature (for 
high values of w). It is: 

I. The theoretical conductor of constant resistance. 

II. The tungsten lamp filament. 


ELECTRIC ILLUMINANTS Tt? 


III. The osmium lamp filament. 

IV. The metallized carbon, or gem lamp filament. 

V. The treated carbon, or 3.1-watt carbon-filament lamp. 
VI. The untreated carbon, or base filament. 


Such a conductor, which fulfils the conditions, 6 >0, can be 


ha 
SRR Saas 
mess 

itl calles 


— Oa 
\ Re 










Resistance 


Fig, 2.—Resistance-Temperature Characteristics of Incandescent Lamp 
Filaments. 


operated satisfactorily on constant-potential as well as on constant- 
current circuits, provided, obviously, that its resistance is chosen 
so as to consume the rated power at the constant voltage respectively 
current of the circuit; on constant-potential supply the current, 
and thereby the power consumed by the conductor, is limited to 


118 ILLUMINATING ENGINEERING 


that corresponding to the supply voltage; on constant-current 
supply the terminal voltage, and thus the power consumed by the 
conductor, is limited to that corresponding to the supply current. 


4. Volt-Ampere Characteristic of Pyroelectrolytic Conductors. 
The Nernst Lamp Glower as Pyroelectrolyte. The Insta- 
bility range, “© 
tial Supply, and the Necessity of pane Resistance or 
Reactance. The Nernst Lamp 


<0, of Pyroelectrolytes on Constant-Poten- 


Very different are the conditions in the conductor of the Nernst 
lamp, the Nernst lamp glower. This belongs to a class of con- 
ductors, the pyroelectrolytes, in which the temperature coefficient 
within a certain range of temperature, and thus of current, is so 
greatly negative, that with increase of current the terminal voltage 
decreases. That is, with increase of temperature the resistance 
drops faster than the increase of current required to produce the 
increase of temperature, and the voltage e=ir thus decreases with 
increase of i. In this range, it therefore is: 


Such pyroelectrolytic conductors are many metal oxides, silicates, 
sulphides, ete. A typical volt-ampere characteristic of such a con- 
ductor (magnetite) is given in Fig. 3, with Vi as abscissae,* the 
terminal voltage e as ordinates. As seen, from i=0 to i, it is 
ee >0; from 1, tons is 2 <0, and for i>1, it is again Se >0. 
With most pyroelectrolytes the voltage peak at i, is so high that 
the conductor cannot be carried beyond it by the mere application 
of voltage, but artificial heating is required, and the resistance 
below i, is usually extremely high, usually near i, fusion occurs, 
and beyond that the conductor is an ordinary electrolytic con- 
ductor. 

The operating point of the Nernst glower is in the range between 

d 


° . e 
i, and i,, where a ~<" 
i 


* For the purpose of better showing the initial part of the curve, Vi 
is used as abscissae, instead of i. 

+ See “ Electric Conduction,’ paper read before the Hiectrochemicat 
Society, 1907, by the author. 


ELECTRIC ILLUMINANTS 119 


A conductor, in which 2% <0, can be operated on constant- 


current supply, but cannot be operated on constant-voltage supply ; 
but at constant terminal voltage it is unstable within the entire 
range from i, to i,, in Fig. 3; on constant-voltage supply an in- 
crease of current, by lowering the voltage consumed by the con- 
ductor, causes a further increase of current and power, and thus 
further decrease of voltage, increase of current and power, etc., 






ye Arya ae fag EETS 1SCIC 
{ee | 
| | /é Cai Cs 


Low-Resistance Fyroelectrolyle 









a fpere 





Fie. 3. 


and the conductor destroys itself by melting; a slight decrease of 
current causes an increase of the voltage required by the con-_ 
ductor, and since this is not available on constant-voltage supply 
a still further decrease of current, increase of required voltage, 
ete., and the conductor open-circuits, that is, the lamp goes out. 
On constant-potential supply, such a conductor therefore either 
open-cirecuits or short-circuits, and to operate it at constant power 
on a multiple circuit a resistance or reactance is required in series 
5 


120 ILLUMINATING ENGINEERING 


to the pyroelectrolyte sufficiently large so that the voltage consumed 
by pyroelectrolyte (glower) plus steadying resistance increases 
with increase of current, that is, fulfils the conditions of operation 
de 
di seat 

The Nernst lamp thus requires a ‘‘steadying resistance” in 
series to the glower. To reduce this resistance, and thereby the 
waste of power caused by it, to a minimum, iron wire is used, 
operated in hydrogen or in a vacuum at that range of tempera- 
ture at which the temperature coefficient of the iron is abnormally 
high, and with increase of the current i the resistance r very 
rapidly increases, thus causing an abnormally rapid increase of ir. 

In arc- and Geissler-tube conduction, a similar instability on 
constant potential will be discussed. 


on constant-potential supply, 


5. The Inght Radiation of Solid Conductors, as Incandescent 
Lamps and the Nernst Glower. Black-Body, Gray-Body and 
Colored-Body Radiation. Effect on the Efficiency of the 
Incandescent Lamp Filaments and the Nernst Glower. Limi- 
tation of Efficiency 


The light production by solid conductors as radiators is tempera- 
ture radiation. That is, by the resistance of the conductor, the 
electric power i’r is converted into heat, causing a rise of tempera- 
ture which produces the radiation. 

Normal-temperature radiation, that is, black-body or gray-body 
radiation, as given very closely by the various types of carbon- 
filament lamps, is a very inefficient light producer. The efficiency 
of ight production increases with increase of temperature, but is 
still very low at the highest temperatures at which solids can be 
operated. The selective radiation of a colored body which is de- 
ficient in radiating power in the ultra-red gives a higher efficiency 
of light production. The radiation of some of the metal filaments, 
and that of the Nernst lamp glower, is such a colored-body radia- 
tion, and thereby gives a light efficiency higher than corresponds 
to the temperature of the radiator. However, the selectivity seems 
to decrease with increase of temperature, that is, with increasing 
temperature the body seems to approach more a gray body. Yor — 
instance the Nernst glower radiates strongly selective at low tem- 
perature, at its operating temperature the radiation curve has 


ELECTRIC ILLUMINANTS 121 


greatly smoothed out,* and while there is probably a gain in 
efficiency in some metal filaments and the Nernst glower over 
normal-temperature radiation, the gain does not seem to be so 
large as to bring the efficiency of light production much beyond 
that reached by normal-temperature radiation, and it does not 
appear probable that we shall be able to reach very much higher 
efficiencies by colored-body temperature’ radiation. 


6. Relation of Refractoriness and Vapor Tension or Disintegra- 
tion to the Possible Efficiency af the Incandescent Lamp. 
Comparison of the Carbon Filaments with the Metal Fila- 
ments. 


Since temperature radiation reaches fair values of light efficiency 
only at very high temperatures, only the most refractory bodies 
come into consideration as radiators in incandescent lamps. 

The most refractory substances are carbon, tungsten, osmium, 
tantalum, ete.t 

However, refractoriness is not the only requirement, but the 
vapor tension, or rate of disintegration of the material below the 
melting point, is equally of importance, since on it depends how 
far we can, in the operating temperature of the radiator, approach 
its melting point. This is well illustrated by the relation between 
tungsten and the different forms of carbon.§ 

Carbon is the most refractory body, and has been the first em- 
ployed in commercially successful incandescent lamps, and the 
carbon-filament lamp still is the one used in the largest quantities. 
Carbon has the disadvantage of a relatively rapid evaporation or — 
disintegration far below its boiling point, and this limits the oper- 
ating temperature of the carbon filament so that we cannot get 
the full benefit of the high refractoriness of carbon; but metals, as 
tungsten, which are less refractory than carbon, can give a higher 
efficiency by being operated at higher temperature. Great differ- 
ences in stability, however, exist between different modifications - 
of carbon. 


* Bulletins of the National Bureau of Standards. 

+ See “ Radiation, Light and Illumination,” by the author, p 70. 
{See “ Radiation, Light and IJlumination,” p. 77. 

§ See “ Radiation, Light and Illumination,” p. 79. 


122 ILLUMINATING ENGINEERING 


7. The Production of the Carbon-Filament Lamp. Base Carbon 
and Treated Carbon, and their Stability 


The first commercial carbon-filament incandescent lamps were 
made of carbonized bamboo fiber. Very soon this was replaced by 
the squirted filament, which could be produced more uniformly. 
A solution of cellulose in zine chloride (or cupric ammon), or of 
nitro-cellulose in glacial acetic acid, is squirted through a fine 
hole into a hardening solution: methyl alcohol with zinc-chloride 
solution, diluted acid with cupric-ammon solution, water with 
nitro-cellulose. The filament is then washed, put into the desired 
shape (in the case of nitro-cellulose, after reduction to cellulose) 
and dried. It then consists of a structureless cellulose, in appear- 
ance very similar to horn. This is now carbonized in a gas furnace 
at high temperature, and constitutes what is now known as a “ base 
filament,” because it is mainly used as a base on which to deposit 
a better form of carbon. The base carbon is not very stable at 
high temperature, and early lamps made of it, therefore, had only 
a relatively low efficiency. It has a high resistance and a high 
negative-temperature coefficient, as shown by its characteristic in 
Figs. 1 and 2. Somewhat later a considerable improvement in 
efficiency resulted from the introduction of the “ treated filament.” 
The base filament is electrically heated in an atmosphere of hydro- 
carbon vapor (gasolene) in a vacuum, and by the dissociation of 
the vapor a shell of a different modification of carbon is deposited 
on the base. This shell carbon has a far greater stability at high 
temperature, thereby allowing the operation of the lamp at higher 
temperature and thus higher efficiency. It is of lower resistance, 
and in the treated filament lamp most of the current thus flows 
in the shell; less in the inner core or base of the filament. The 
temperature coefficient of the shell carbon is still negative, but 
decreases with increasing temperature, and finally begins to rise, 
so that the compound structure of the treated filament gives a 
characteristic as shown in Fig. 2. 


&. Metallized Carbon, its Resistance and Temperature Coefficient, 
and the Gem Lamp 


A few years ago a further advance was made by discovering a 
form of carbon of still much higher stability, the metallized carbon 
used in the so-called “gem lamp.” The shell carbon (but not 


ELEcTRIC ILLUMINANTS 123 


the base carbon) converts at the highest temperature of the electric 
furnace into a modification of carbon of nearly metallic character ; 
it has a very low resistance, lower than some metals, and a positive- 
temperature coefficient, like metals, though lower than that of 
pure metals, as shown by the characteristic of the carbon filament 
with metallized shell, in Figs. 1 and 2. In the production of the 
gem lamp the base filament is heated in the electric furnace to 
expel all impurities, then treated in gasolene vapor, and thereby 
a layer of shell carbon deposited on it, and then is once more heated 
in the electric furnace. The filaments are then sealed in glass 
bulbs with platinum leading-in wires and exhausted. It gives an 
efficiency of about 3.3 watts per candle-power. 

Apparently, the electric resistance and its temperature coefficient 
are indications of the stability of carbon at high temperature; 
the lower the cold resistance and the higher its temperature co- 
efficient the more stable is the carbon at high temperature, and the 
higher efficiencies can thus be reached. 


9. Metal-Filament Incandescent Lamps. Osmium Lamp, Tantalum 
Lamp, Tungsten Lamp. Their Efficiencies 


In recent years metal-filament incandescent lamps have been 
developed, and are rapidly replacing the carbon-filament lamps by 
their higher efficiency. 

First, the osmium-filament lamp was developed, giving an effi- 
ciency of about 1.9 watts per candle-power. Its filament was made 
by some squirting process, similar to the carbon filament. It 
found a limited use only, since osmium is a very rare metal, exist- 
ing in very limited quantities, and was soon replaced by the tanta- 
lum filament. Tantalum is a ductile metal, and the tantalum 
lamp is made by winding drawn tantalum wire on a glass frame. 
The tantalum lamp gives an efficiency of about 2.6 watts per 
candle-power, hence lower than the osmium lamp but higher than 
the gem lamp. Tantalum, while a rare metal, exists in fairly large 
quantities, and the tantalum lamp appeared very promising until 
the development of the more efficient tungsten lamp of 1.5 to 1.7 
watts per candle-power. 

The tantalum lamp was the first incandescent lamp made of 
drawn metal, and showed the features of a much better life with 
direct current than with alternating current; with alternating cur- 


124 ILLUMINATING ENGINEERING 


rent the drawn filament loses its ductility and gradually offsets, 
that is, breaks up into numerous short lengths, which are welded 
together. 


10. The Manufacture of the Tungsten Lamp 


The highest efficiencies of incandescent lamps have been realized 
by the tungsten filament. Tungsten, or wolfram, is a fairly com- 
mon metal; is extremely refractory, more than osmium or tantalum, 
but less than carbon, but fairly difficult to produce in such purity 
as necessary as filament.* Several methods of manufacture of 
tungsten filaments have been devised and are still in commercial 
development, though many millions of tungsten lamps have been 
made. One series of processes consists of squirting the metal as 
powder, or in the colloidal state, with some binder, and then burn- 
ing out the binder by electric heating in a suitable gas; another 
by squirting a filament of tungsten oxide with some reducing ma- 
terial, reduce by heat, and then eliminate the excess of reducing 
material and of oxide by electrically heating in a suitable gas at 
reduced pressure. A third process consists of squirting or drawing 
a wire of some tungsten alloy, and by electrically heating evaporate 
the alloying metal and sinter the tungsten, and, finally, methods 
have been found to draw the pure tungsten metal into wire of 
sufficiently small size for use in filaments. All these methods 
except the last give a filament which is not ductile, but brittle, like 
the osmium and carbon filament, and, therefore, due to its ex- 
treme thinness, is very fragile. 


11. Thinness and Length of Metal Filaments. Fragility 


All these metals have a much lower resistance than the base 
carbon, which constitutes the main part of the carbon filament, 
and since they are more efficient, that is, at the same supply voltage 
require less current for the same light flux, they must be of ex- 
treme thinness and considerable length. Therefore, in these lamps 
a number of squirted filaments are used in series, or with drawn 
wire a considerable length of wire wound zigzag on a frame. 
This difficulty does not exist with the metallized carbon filament ; 


* For instance, a contamination by 0.3 per cent of carbon would rep- 
resent an impurity of 10 per cent tungsten carbide Wo,C. 


ELEctrRIc ILLUMINANTS 125 


while the metallized carbon also has a very low resistance, it is 
used only as a thin shell on the base carbon, which practically does 
not carry any current, in the gem lamp, while the metal filaments 
are solid conductors in which the whole cross-section conducts. 


12. Hfficiencies of the Different Incandescent Lamps. Conven- 
tional Rating in Horizontal Candle-Power. Relation of Effi- . 
ciency to Useful Infe 


The approximate efficiencies, or rather specific consumptions, of 
the different types of incandescent lamps are: 


Base carbon filament (not used any more).......... 5 watts per c. p. 
Serre ET OLE OST ALIA TTLETI G2 ewe ncaa. 6 6-0: cd 0 0:9 0 < eles aueie ele “ 

Metallized carbon (gem filament)..... Uy RST Ie a.0 

TCP PT TMMNUPL TIS te iota cls, ecu cin os reese vce eneeaseye 2.6 

eee IP Ree ts, ce + sss ces se dase es wesc 1.9 


Pare pee TIMBERS TINE, Ses oc arslels wieks sidtiw eds ces sees Sees 1b etor 1.7 * 


Light flux is measured in lumens, and light efficiency thus in 
lumens per watt, specific consumption in watts per lumen. Usually 
instead of the lumen as measure of the ight output of an illumi- 


nant the mean spherical candle-power is used, which is rik times 
Tv 


as much, and the efficiency then given in mean spherical candles 
per watt, the specific consumption in watts per mean spherical 
candle. 

By convention, incandescent lamps are usually rated in mean 
horizontal candles, and their specific consumption expressed by 
giving the watts per mean horizontal candles and the spherical 
reduction factor: Thus, above lamps are commercially rated at: 


Treated carbon filament...3.1 watts per mean horizontal candle-power 


Poy ty pp eee a re 2.6 watts per mean horizontal candle-power 

Vemtalum Wamp eee os 3k 2.0 watts per mean horizontal candle-power 

OBMHUIM JAM Do kic aie. 1.5 watts per mean horizontal candle-power 

PRR OTLCTULIATND: (5 oie dey ss 1.15 to 1.83 watts per mean horizontal candle- 
power 


At the spherical reduction factor 0.78, this gives above values. 
In comparison with other illuminants, obviously, the horizontal 
candle-power has no meaning, but the total flux of light, that 1s, 
the mean spherical candle-power, has to be used. 


* See “ Radiation, Light and Illumination,” p. 179. 


126 ILLUMINATING ENGINEERING 


When considering efficiency, however, the useful life of the lamp 
must also be considered. Obviously, higher or lower efficiencies 
_ may be reached by operating the same lamp at higher or at lower 
voltage. 

When speaking of the efficiency of a carbon-filament lamp it is 
understood, by general convention, that the lamp is operated at 
such a voltage as to give a useful life of 500 hours. As useful life 
is understood the time during which the lamp, on constant-voltage 
supply, decreases by 20 per cent in candle-power.* 

With metal filaments no such convention has yet been generally 
established, but due to the higher efficiency and higher cost of 
the lamp probably a useful life of 1000 hours or more will be 
economical. 

Efficiency tests of incandescent iat therefore are meaningless 
if not accompanied by life tests at that efficiency. 


18. Relation of the Efficiency of the Incandescent Lamp to the Size 
of the Unit or the Power Consumption. Limitation by Sup- 
ply Voltage at Small Units, by Size of the Lamp Globe at 
Large Units. Wide Range of Umts with Fairly Uniform 
Efficuency 


Characteristic of the incandescent lamp is, that its efficiency is 
(theoretically) independent of the unit of light; filaments of large 
diameter and great length, consuming large power and giving a 
large unit of light, give the same efficiency when operating at the 
same temperature as filaments of small diameter and short length, 
that is, filaments which consume small power and give small units 
of hight, and operating at the same temperature, should have the 
same life. Thus incandescent lamps give a wide range of sizes 
of illuminants of nearly the same efficiency. 

A limitation of the possible size of incandescent light units 
appears with small sizes in the voltage of the system of electric 
power-supply. At the same supply voltage—110 or 220—a smaller 
light unit requires a filament of smaller diameter, and finally a 
point is reached where the small diameter makes the filament so 
delicate that either the life of the lamp would be materially short- 


* See “ Radiation, Light and Illumination,” p. 79. 
+ See “General Lectures on Electrical Engineering,’ by the author, 
p. 209. 


ELEctTRIC ILLUMINANTS 127 


ened, or a lower operating temperature, that is, lower efficiency, 
must be allowed. Thus, with the carbon-filament lamp on 110- 
volts supply, 50 watts (or 16 horizontal candle-power with the 
treated filament, 20 horizontal candle-power with the gem fila- 
ment), are the smallest units at which full efficiency can be reached. 
Carbon-filament lamps of less than 50 watts for 110-volt circuits, 
therefore must be made for lower efficiency, and the efficiency low- 
ered the more the smaller the unit is. Obviously, for a 55-volt 
circuit, an 8-candle-power lamp could be made of the same effi- 
ciency as the 16-candle-power lamp on the 110-volt circuit, and 
_ the 220-volt, 16-candle-power lamp cannot be built any more for 
the same efficiency as the 110-volt lamp, other things being equal. 

The same applies still more to metal-filament lamps, as in these 
the filaments are thinner and longer than in carbon-filament lamps 
of the same voltage and candle-power. Thus in the tungsten lamps 
higher efficiencies are given to the larger units. 

For low-voltage lamps, obviously, this limitation of minimum 
size, by the mechanical structure of the filament, does not exist, 
and lamps of 1- or 2-watts consumption, or even less, at 4- to 10- 
volts supply, can be made of the same efficiency as the 50-watt lamp. 

With increasing size of the unit, a practical limitation is also 
reached; the useful life of the carbon-filament lamp is limited 
largely by the blackening of the globe by carbon deposits, and to 
give equal blackening the surface of the lamp globe should be 
proportional to the power consumed in the lamp. This, however, 
gives for large units impracticably large globes, and the use of 
smaller globes leads to a shorter life. 

This limitation exists less with metal-filament lamps. In these 
it seems that the life is not so much limited by the gradual black- 
ening of the globe as by impairment of the vacuum, and for equal 
performance only the volume of the globe and not the surface, as 
with the carbon filament, should increase proportional to the power 
consumption. This makes metal-filament lamp units of several 
hundred watts feasible, while carbon-filament lamps of such power. 
consumption are impracticable. 

The gem lamp, due to the metallic properties of the filament, 
stands intermediate between the treated carbon filament and the 
metal filament in this respect, and lamp units of 250 watts have 
been fairly successful. 


bes ILLUMINATING ENGINEERING 


14. Inferiority of the Incandescent Lamp in Efficiency to the Flame . 
Arc and Luminous Arc. Superiority in Small Units. Main 
Field of Application of Incandescent Lamps and Nernst 
Lamps in Small Units where no Other Efficient Electric Il- 
luminant Haists 


The incandescent lamp thus gives units of light, of practically 
the same efficiency, from a fraction of a candle-power to several 
hundred candle-powers, covering a wider range than any other 
electric illuminant. 

However, the efficiency of light production is of lower magnitude 
than that of some other electric illuminants; even in the most 
efficient incandescent lamp, the tungsten lamp, the specific con- 
sumption of 1.5 to 1.7 watts per candle is of far higher magnitude 
than the specific consumption reached in some flame arcs and 
luminous arcs, of half a watt or less per candle-power. 

Thus, in efficiency, the incandescent lamp cannot compete with 
the flame are or the luminous arc, and is therefore excluded from 
economical use in those cases where these ares can be used, but 
must find its field of application in those cases where the more 
efficient illuminants cannot be used, and especially is this the case 
with smaller units of light, since the efficiency of the are rapidly 
decreases with decreasing power consumption, while that of the 
incandescent lamp remains the same, and the incandescent lamp 
(including the Nernst lamp) is therefore the only one available 
for smaller units of light, of 100 candle-power or less. 


GASEOUS CONDUCTORS 


15. Difference between Disruptive- or. Geissler-Tube Conduction 
and Continuous or Arc Conduction 


Two forms of conduction of gases or vapors exist: disruptive- 
or Geissler-tube conduction, and continuous or are conduction. The 
distinction is, that in the former the gas which fills the space is 
the conductor; in the latter conduction takes place by a moving 
stream of electrode vapor. Gas or vapor conduction is accom- 
panied by luminescence of the conductor, and thus can be used 
for light production. In Geissler-tube conduction the light gives 
the spectrum of the gas which fills the space between the electrodes ; 
in are conduction the spectrum is that of the electrode material.* 


* See “ Radiation, Light and Illumination,” p. 98. 


ELECTRIC ILLUMINANTS 129 


The conductor may be at atmospheric pressure, as in the carbon 
arcs, flame arcs and most luminous ares; or in a vacuum, as in 
the Geissler tube or the vacuum are (of which the only industrially 
important exponent is the mercury arc). 


GEISSLER-TUBE CONDUCTION 


16. Electrical Characteristics of Geissler-Tube Conduction: Total 
Voltage, Terminal Drop and Stream Voltage as Function of 
Gas Pressure 


Very little is known on the electrical characteristics of Geissler- 
tube conduction. The only commercial illuminant of this class is 
the Moore tube. 


717 Hg PLESSULE, fr 


ie ca Pa 
Sided les ips ofc || 
Se Save 
ARS GPa 





Fig. 4.—Volt-Pressure Characteristic of Geissler Tube. 


It seems that, at constant temperature and constant gas pressure, 
the voltage consumed by the Geissler tube is approximately constant 
de 
di 
characteristic of the Geissler tube thus would be a straight horizon- 
tal line. As result hereof, a Geissler tube cannot be operated on 
constant-supply voltage, but requires a steadying resistance or re- 





and independent of the current, that is, =0. The volt-ampere 


actance to fulfil the conditions of stability, 2 >0. The reactance 


of the step-up transformer is used for this purpose in the Moore 
tube. 


130 ILLUMINATING ENGINEERING 


The voltage consumed by the Geissler tube consists of a potential 
drop at the terminals, the “terminal drop,” and a voltage con- 
sumed in the luminous stream, the “ stream voltage,’ which latter 
is proportional to the length of the tube. Both greatly depend on 
the gas pressure, and vary with varying gas pressure in opposite 
directions: with increasing gas pressure the terminal drop de- 
creases and the stream voltage increases, and the total voltage 
consumed by the tube thus gives a minimum at some definite gas 
pressure. This pressure of minimum total voltage depends on 
the length of the tube, and the longer the tube is the lower is 
the gas pressure of minimum total voltage. 


2 ae 
i powe tnt tl Pressure, fr. Vo 


cH SS ozaw 





Fie. 5.—Volt-Pressure Characteristic of Geissler Tube. 


In Fig. 4 is shown the voltage-pressure characteristic, at constant 
current of 0.1 and of 0.05 ampere, of a Geissler tube of 1.3 cm. 
diameter and 200 cm. length, using air as conductor; and in Fig. 5 
the characteristic of the same tube with mercury vapor as con- 
ductor.* Figs. 4 and 5 also show the two component voltages, 
the terminal drop and the stream voltage. As abscissae are used 
the logarithms of the gas pressure, as measured by McLeod gauge 
at the moment of taking current and voltage readings. 


*It is interesting to note, that total voltage, terminal drop and 
stream voltage in the Geissler tube using mercury vapor as conductor, 
are nearly the same as with air, and entirely different from the terminal 
drop and the stream voltage of the vacuum mercury are. The spectrum 
is the same, the mercury spectrum. 


ELECTRIC ILLUMINANTS yh i 


With increasing pressure the discharge finally stops, due to the 
limited supply voltage; with decreasing pressure, finally the gas 
density becomes so low that a tendency to are conduction appears, 
and the beginning of arc formation usually destroys the tube. 


17. Performance, Lfficiency and Color of Light. The Moore Tube 


As seen, the values of terminal drop are very high, and as this 
voltage gives no equivalent of light, efficiency requires the use of 
such a long tube as to make the terminal drop a small part of the 
total voltage. In consequence hereof, the Moore tube is a very 
large unit of light and does not allow economical subdivision. It 
requires high-voltage alternating current, which is usually pro- 
duced by a step-up transformer attached to the terminals of the 
tube. Intermittent direct current may equally well be used, but 
continuous direct current is not suitable, as the Geissler-tube con- 
duction rapidly changes to arc conduction, and as the latter re- 
quires much lower voltage, leads to short-circuit. 

In the Geissler tube the terminals disintegrate and the gas 
pressure falls fairly rapidly, possibly by absorption of the gas by 
disintegrated electrode material. As commercial illuminant, the 
Geissler tube therefore requires means of feeding gas intermittently 
into the tube. This is done in the Moore tube by an automatic 
valve. 

As far as known, the most efficient Geissler-tube conductor is 
nitrogen. It gives a reddish-yellow light, of an efficiency which 
in very long tubes reaches values of 2.5 watts per candle-power, 
that is, about the same as the tantalum lamp, but of lower magni- 
tude than the flame arc and the luminous arcs. Carbon dioxide 
CO, is also used as conductor. It gives a white light, but a lower 
efficiency. Mercury vapor gives it green light, but also at low 
efficiency. 

The great advantage of the Moore tube is its low intrinsic bril- 
liancy, and in the CO, tube its white color. 


ARC CONDUCTION 


18. Nature of the Arc Conductor. The Arc as Unidirectional Con- 
ductor. Rectification by the Arc. The Alternating-Current 
Arc. Constant-Pressure and Varying-Pressure Ares. 


In the electric arc the current is carried across the space between 
the electrodes or arc terminals by a stream of electrode vapor which 


132 ILLUMINATING ENGINEERING 


issues from a spot on the negative terminal, the so-called negative 
spot, as a high-velocity blast (probably of a velocity of several 
thousand feet per second). If the negative terminal is fluid the 
negative spot causes a depression, which is in a more or less rapid 
motion, depending on the fluidity. Before are conduction can take 
place the vapor stream has to be produced, that is, an are has to 
be started. This is done by bringing the electrodes into contact 
and then separating them, or by a high-voltage spark or a Geissler 
discharge, or by the vapor stream of another arc, or by heating the 
space between the electrodes, for instance, by an incandescent 
filament.* 

The are stream is conducting only in the direction of its motion, 
that is, any body which is reached by the are stream is conductively 
connected with it, if electro-positive regards to it, but is not in 
conductive connection if negative or isolated. The are thus is a 
unidirectional conductor, and as such has found an extensive use 
for the rectification of alternating current.f 

Since the arc is a unidirectional conductor, it usually cannot 
exist with alternating current, since at the end of every half wave 
the vapor stream extinguishes, and at the beginning of the next 
half wave a new vapor stream in opposite direction has to be 
started. An alternating-current are exists only if the conditions 
are such that at every half wave a new arc starts. ‘This is the 
case if the voltage in the circuit is sufficiently high to send a dis- 
ruptive spark across the gap at every half wave, or if the arc 
temperature is so high as to start the are, as is the case with the 
carbon are. 

In their industrial application we may distinguish between con- 
stant-pressure arcs and varying-pressure arcs, that is, arcs in an 
enclosed space, usually a vacuum, in which the gas or vapor pres- 
sure varies with the current, etc. ‘The only industrially used are 
of the latter class is the mercury are. 


* See “ Radiation, Light and Illumination,” p. 106. 

+ On the arc as unidirectional conductor, see “‘ Radiation, Light and 
Illumination,” p. 111. On the electric characteristics of the mercury 
arc rectifier, see “Theory and Calculation of Transient Electrical 
“Phenomena and Oscillations,’ by the author, p. 249. 

~See “Radiation, Light and [lumination,” p. 115. 


ELECTRIC ILLUMINANTS 133 


CONSTANT-PRESSURE ARCS 
19. Volt-Amperes and Volt-Length Characteristics of the Are, 


de 
di aa. 


















JSR SSS 
Sees eee 
‘J SS BES See eee 
|  S@E Sea Ree 
“SE SSSan a 









ee ee 


Fig. 6. 


Characteristic of the arc as conductor is, that the voltage de- 


creases with increase of current, that ig 2¢ <0 over the entire 


di 
range. ‘The volt-ampere characteristics of the are therefore are 
curves of fhe shape shown in Fig. 6 for the magnetite arc, for the 


134 ILLUMINATING ENGINEERING 


arc lengths of 0.5, 1.5 and 2.5cm. With increasing current the 
are voltage decreases and approaches a finite limiting value, which 
with the magnetite arc is about 30 volts (about 36 volts with the 
carbon arc, 13 volts with the mercury arc, ete.). Inversely, with 
decreasing current the voltage increases, and tends towards infinity, 




























eee 
A et 
Ga ee | 





















or rather probably the voltage required by the electrostatic spark, 
that is, by Geissler-tube conduction across the are gap. 

At constant current, with increasing are length, the are voltage 
increases very nearly proportional to the are length, and the volt- 
length characteristics of the are thus are practically straight lines, 
as shown in Fig. 7 for the magnetite arc of 2, 4, 8 and 16 amperes.* 


* See “‘ Radiation, Light and Illumination,” p. 137. 


ELEcTRIc ILLUMINANTS 135 


20. Dependence of the Arc Voltage on Two Independent Variables, 
Current and Arc Length, Instability of the Arc on Constant- 
Voltage Supply. Necessity of Steadying Resistance or Re- 
actance. The Stability Curve of the Arc 


The are as conductor in industrial illuminants thus differs from 
the solid conductors discussed in the preceding by two main char- 
acteristics : 

a. In the solid conductors the relation between e and i is fixed, 
that is, e is determined by i, and inversely. In the arc, however, 
two independent variables exist, the current or voltage and the 
arc length. That is, e is a function of i as well as of 1 which can 
be expressed with fairly good approximation (except for very small 
currents, for which the voltage is higher than given by the equa- 
tion) by the formula: 
e(1+38) 

ML. 
where e,, c and 6 are constants, depending on the material of the 
electrodes, and more particularly on the negative electrode. 

Least close is the agreement with above formula in the carbon 
arc, Which in many other properties shows an exceptional character 
as result of the physical properties of carbon.* 


b. In the are it always is oe 





SES ENG 





<0, while in the incandescent-lamp 
filaments it is 2 >0, 


Herefrom follows: 

An arc is unstable and cannot be operated on constant-voltage 
supply, but with constant voltage at the arc terminals a slight 
momentary increase of the arc resistance, by requiring a higher 
voltage, decreases the current and thereby still further increases 
the required voltage and the arc goes out. Or, a slight momentary 
decrease of the are resistance increases the current, thus lowers the 
arc voltage, thereby, at constant-supply voltage, increases the cur- 
rent and still further lowers the are voltage, etc., and the are 
short-circuits. The arc, however, is stable on constant-current 
supply. 

The are thus is essentially a constant-current phenomenon, its 
operation more steady on constant-current circuits, and additional 
apparatus is required for its operation on constant-potential cir- 


* See “ Radiation, Light and Illumination,” p. 140. 


136 ILLUMINATING ENGINEERING 


cuits. That is, a resistance or reactance (with alternating arcs) 
must be inserted in series sufficiently large so that for the total 


voltage consumed by the are with its steadying resistance S2 >0. 


Thus, while in Fig. 8 the lower curve is the volt-ampere char- 


StaAbiliy —CNAr ACCEL IS UE 


LS crm L engtl 


aE 





Fia. 8. 


acteristic of a 1.5 cm. magnetite arc, to operate such an arc on a 
constant-potential supply a much higher voltage is required: the 
supply voltage must be greater than that given by the upper curve 
in Fig. 8 to give stable operation, and the more so the greater the 
required stability. This curve thus is called the “ stability curve” 
of the are.* 


* See “ Radiation, Light and Illumination,” p. 142. 


ELEctTrRIic ILLUMINANTS Loy 


21. Instability of Parallel Operation of Arcs Without Steadying 
Resistances. Instability Due to Non-Inductive Resistance 
Shunt. Hatinction by Shunted Capacity. The Arc as In- 
terrupter. The Singing Arc 


From the characteristic of the areoe <0 also follows: 


Several arcs cannot be operated in parallel except by giving 
each of them a steadying resistance or reactance as large as would 
be required for its operation on constant-potential circuit. With- 
out this all the arcs go out but one. | 

Shunting the arc by a non-inductive resistance decreases its 
stability, and with decreasing resistance a definite value is reached 
at which the arc becomes unstable, that is, goes out. The stability 
of an arc thus can be measured by the current which can be shunted 
around it by a non-inductive resistance. 5 

A condenser in shunt to the are makes it unstable and interrupts 
it; a momentary increase of arc resistance, and thereby increase of 
arc voltage, increases the current shunted momentarily by the 
condenser, thereby decreases the are current, and still further in- 
creases the are voltage and shunts still more current into the con- 
denser, etc. Even a small condenser in shunt to the are thus puts 
it out. If the supply voltage is sufficiently high to restart the are, 
after it is put out by a shunted condenser, the arc with shunted 
condenser then acts as an interrupter, causing rapid successive 
interruptions of the circuit with fairly constant frequency. The 
lower the stability of the arc the more sudden are the interruptions, 
and low-temperature arcs, as the mercury arc, thus give inter- 
ruptions of extreme suddenness. Inversely, if the capacity is very 
small and the gas filling the space around the are stream of low 
dielectric strength, as hydrogen or light hydrocarbons, the are 
may start again, through the residual arc vapor, before completely 
extinguished, and the arc current becomes pulsating, the so-called 
“singing arc.” 


22. Stream Voltage and Terminal Drop of the Arc. Heating of 
the Terminals by the Terminal Drop. The Carbon Arc as 
Incandescent Radiator. Relation between the Efficiency of 
the Carbon Arc and the Size and the LIfe of the Terminals 


Voltage, and therefore power, is consumed in the are stream and 
at the arc terminals. The power consumed in the are stream is 


138 ILLUMINATING ENGINEERING 


converted, more or less directly, into radiation, and if a large part 
of this radiation is in the visible range, as is the case with titanium, 
calcium and mercury vapor as conductors, the are stream may be 
used as illuminant. If very little of the radiation is in the visible 
range—as is the case with carbon vapor as conductor—the arc 
stream does not contribute appreciably to the light given by the 
lamp. 

The power consumed at the electrodes is partly converted into 
the latent heat of evaporation and the kinetic energy of the moving 
vapor stream (which is the are conductor) largely into heat, 
especially at the positive terminal. If the arc terminals then 
are sufficiently small to reduce the heat conduction away from 
them, and of sufficiently refractory material to reach very high 
temperature, they may be used as radiators in giving light. The 
radiation then is due to incandescence or temperature radiation. 

The latter is the case with the plain carbon arc lamp. When 
using pure carbon as arc-lamp electrodes the arc stream gives very 
little light, and that of a useiess, violet color. Considerable heat 
is, however, produced at the positive electrode, and if this is not 
too large its tip reaches a very high temperature: the boiling point 
of carbon, and then gives light by temperature radiation, practically — 
black-body radiation. The plain carbon arc therefore gives light 
by incandescence, just like the carbon-filament incandescent lamp, 
and the are stream in the former is merely the heater which raises 
the temperature of the radiator, the positive-electrode tip, to a 
high temperature, and the much higher radiation efficiency and 
white color of the carbon arc, compared with the carbon filament, 
is due to the higher temperature of the former. Nevertheless, 
while the radiation efficiency of the carbon arc is the highest which 
can be reached by black-body radiation, it is very much lower than 
the efficiencies available by luminescence of the are stream. 

Of the heat produced at the positive terminal of the carbon are. 
only a part becomes useful as incandescent radiation; the rest is 
conducted away through the electrode, carried away by air currents, 
etc. The lower this loss, that is, the smaller the electrodes, the 
higher is therefore the efficiency, and with very large electrodes 
the heat conduction becomes so large that the electrode tips do not 
reach any more the temperature of efficient radiation, and the 
efficiency vanishes. The efficiency of the carbon are lamp thus 
depends on the size of the electrodes, and increases with decreasing 


ELEcTRIc ILLUMINANTS 139 


size. However, with decreasing size, the consumption of the elec- 
trodes by combustion increases, and thus requires more frequent 
trimming of the lamp, that is, higher cost of maintenance. 


23. The Open Carbon Arc or Short-Burnming Arc Lamp. The 
Enclosed Carbon Arc or Long-Burning Lamp. Its Infervority 
in Efficiency 


The first carbon are lamps were operated with high current: 
‘on 9.6 amperes constant direct-current circuits, with electrodes, 
which were fairly small relative to the current, and therefore gave 
fairly good efficiencies: about 1 watt per candle-power. However, 
under these conditions, the rate of consumption of the electrodes 
was very rapid, and electrodes of the greatest length, which could 
conveniently be used in a lamp, lasted only a few hours. As result 
thereof, twin carbon lamps were designed, and were in extensive 
use. The high cost of operation, due to the required daily trim- 
ming, of these so-called “ open arc lamps” or “ short-burning are 
lamps” led to the development of the enclosed carbon are lamp. 
In this type of lamp the arc is enclosed in a small, nearly air-tight 
glass globe, and the rate of consumption of the electrodes thereby 
greatly reduced and a longer life of electrodes secured. As the 
retarded combustion of the electrodes resulted in their assuming 
a more flattened shape, the are length had to be increased to limit 
the obstruction of the light issuing from the positive electrode by 
the shadow of the negative electrode. The higher arc voltage re- 
sulting herefrom required a decrease of current to retain the same 
power consumption, and while the open arc operated at 40 to 45 
volts on 9.6 amperes circuits, the enclosed arc lamp consumes 70 
to %5 volts on 6.6 or 7.5 amperes circuits. As the same size of 
electrodes was retained, or the size even increased, to get.a long 
life, while the current and thereby the luminous area of the elec- 
trodes was reduced, the heat losses by conduction and convection 
were greater in the enclosed arc, and the efficiency therefore lower 
than in the open arc. Nevertheless, the advantage of lower main- 
tenance cost resulting from the less frequent trimming, weekly 
with the enclosed arc lamp against daily with the open arc lamp, 
has led to the entire abandonment of the latter, and while open 
arcs have survived in a few cities they have practically ceased as 
an article of manufacture. 


140 ILLUMINATING ENGINEERING 


24. Uneconomical Operation of Continuous-Current Series Arc 
Circuits. The Series Alternating Enclosed Are Lamp. Its 
Very Low Efficiency 


In regard to the electrical-power supply, the enclosed arc lamp 
is inferior to the open are lamp, since with the former the higher 
voltage and lower current gives, with the same maximum Voltage 
of the constant-current circuit, a smaller unit, and with direct 
current an arc machine was required for each circuit. This was 
such an economical disadvantage that the direct-current series 
enclosed carbon are lamp is used to a limited extent only in such 
places where efficiency of light production is essential, and the 
illuminant, which is most universally used for street lighting, is 
the constant-current alternating enclosed carbon are lamp. With 
this lamp, operating from constant-current transformers, the small 
size of the individual arc circuit is not such a serious handicap. 

The economic disadvantage of numerous small machine units, 
which handicapped the series direct-current are lamp, has been 
eliminated by the development of the constant-current mercury 
arc rectifier system, which permits operation of constant-direct 
current are circuits from constant-current transformers. This de- 
velopment, however, was too late to help the direct-current carbon 
arc, but, coming after the development of the luminous are, it led 
to the rapid introduction of the latter in place of the carbon arc. 

The efficiency of the alternating-current carbon are lamp, how- 
ever, is much lower than that of the direct-current lamp: in the 
alternating-current lamp the losses of heat through the electrodes 
are more than doubled: while the heat loss by conduction and con- 
vection is continuous, heat is produced at either electrode mainly 
during that half wave of current where the electrode is positive, 
and then only during that part of this half wave where the current 
is high. Thus, while the alternating-current carbon are lamp gives 
light from both electrodes, its efficiency of light production is much 
lower, and with the standard series enclosed alternating-current 
arc lamp at,70 to 75 volts per lamp, on 6.6 and 7.5 amperes con- 
stant alternating-current circuits, the specific consumption is up 
to 2.5 to 3 watts per candle-power, and even higher, that is, the 
efficiency has dropped down below that reached with modern in- 
candescent lamps. 

In spite of its very low efficiency, the small amount of attention 
required by it, and the convenience of operation from alternating- 


EvEctric ILLUMINANTS 141 


current supply circuits, through constant-current transformers in 
street lighting, has led to the almost universal adoption of the 
alternating-current enclosed carbon are lamp, and probably more 
lamps of this type are used in street lighting than of all other types 
together. 


25. Replacement of the Enclosed Alternating Carbon Arc by the 
Magnetite Arc Lamp in Street Lighting, by the Intensified 
Arc or the Tungsten Incandescent Lamp in Indoor Inghting. 
The Intensified Arc Lamp 
However, with the development of high-efficiency incandescent 
lamps, the position of the standard enclosed alternating carbon are 
lamp became untenable, and while it is still being used in enormous 
numbers it is being rapidly replaced by the magnetite arc lamp 
in street lighting, and by the intensified are lamp and the tungsten 
incandescent lamp in mdoor lighting, and the manufacture of the 
enclosed alternating carbon arc lamp has greatly decreased. 
While thus the enclosed carbon are lamp is rapidly disappearing 
from the streets, before the luminous arc, for indoor lighting, 
where the luminous arc and the flame arc are handicapped by being 
too large units of light, and by producing smoke and gases, and 
the tungsten lamp is the only competitor, the enclosed carbon are 
lamp is retaining its field as the “intensified are lamp.” Since 
the efficiency of the carbon are lamp increases with decreasing size 
of carbons, by the use of very small carbons in an enclosed type 
of lamp, a very good efficiency, about 1 candle-power per watt, is 
reached in the so-called “intensified arc lamp” on direct current 
as well as on alternating current. The life of the electrodes of the 
intensified arc lamp is shorter than that of the enclosed arc lamp 
of old, but as this lamp is mainly used indoors, where usually the 
daily operation is only a few hours, the life is sufficient to reduce 
the frequency of trimming satisfactorily, and the higher efficiency 
and white color of light gives to the intensified arc an advantage 
over the tungsten Jamp in those cases where large units of light 
are permissible. 


26. The Luminous Arc and the Flame Arc. Their Characteristic 
Differences, Advantages and Disadvantages. The Magnetite 

Are 
The carbon arc is an illuminant using a solid radiator and pro- 
ducing light by incandescent radiation, like the incandescent lamps. 


142 ILLUMINATING ENGINEERING 


-In all other arcs luminescence plays an essential part, and all or 
most of the light is given by the arc flame as vapor conductor. 

These luminescent arcs can be divided into two classes: the 
luminous arcs and the flame arcs.* In the luminous arcs the lumi- 
nescent material is introduced into the are stream by electro-con- 
duction from the negative, that is, is used as are conductor. Typical 
arcs of this class are the so-called magnetite are and the mercury 
are. The latter, as vacuum arc, will be discussed later. In the 
flame arcs the luminescent material is introduced into the arc 
stream by heat evaporation, either from the positive as the hotter 
terminal, or from both terminals. The characteristic difference 
resulting herefrom is, that in the luminous are the temperature 
of the electrode has no direct relation to the efficiency, and the 
electrodes thus can be maintained at such low temperature as to 
consume very slowly. The luminous arc thus lends itself to the 
production of long-burning arc lamps, that is, lamps requiring very 
infrequent trimming, and the size of the electrodes is usually made 
such as to give a life of 100 to 200 hours as the longest time which 
it is advisable to allow a lamp to burn without cleaning the globe, — 
and other attention. The positive electrode of the luminous arc 
is entirely immaterial, and usually made of some metal of high 
heat conductivity so as not to consume appreciably, that is, of a 
life of some thousand hours. 

At the same time the number of materials which can be used 
in the luminous are is much more limited, the difficulties of design 
so as to get steady operation, greater than with the flame arc, and 
no successful luminous are has yet been commercially developed 
for alternating-current circuits, but the luminous are has been 
developed for direct-current circuits in the so-called “ magnetite 
arc lamp,” also occasionally called “ metallic-oxide arc lamp” and 
“ferro-titanium are lamp.” In this the negative electrode is a 
mixture of the oxides of iron, titanium and chromium (magnetite, 
illmenite, rutile, chromite), usually enclosed by a thin iron shell. 
The positive electrode is a permanent part of the lamp. 

The magnetite arc lamps are operated on constant direct-current 
eircuits of 4 amperes and of 6.6 amperes, with about 75 volts per 
lamp, usually from constant-current transformers through mercury 
are rectifiers. 


* See “ Radiation, Light and Illumination,” p. 123. 


Ewiectric ILLUMINANTS 143 


27. The Flame Carbon Arc. Relation between Size of Electrode 
and Efficiency. The Short-Burning and the Long-Burning 
Flame Carbon Arc. The Yellow Color of the Flame Carbon 
Arc. Titanium, Calcium and Mercury as the Three Most 
Eficuent Arc-Stream Radiators 


In the flame arcs the luminescent material is introduced into 
.the are stream largely by heat evaporation, a high temperature 
of the positive electrode thus is essential, and, to some extent, 
similar relations exist between the size and therefore temperature 
of the electrodes and the efficiency. Carbon is always used as the 
main electrode material, since carbon gives the hottest arc, and 
also the steadiest arc, and the inherent steadiness of the carbon 
are has made the development of the flame carbon arc lamp less 
difficult and therefore more rapid than that of the luminous are, 
and made it possible to operate such arcs on alternating-current 
circuits as well as on direct-current circuits. 

Since, however, carbon rapidly consumes, and the size of the 
electrodes cannot be materially increased without loss of efficiency, 
the flame carbon arc lamp is essentially a short-burning are lamp, 
requiring daily trimming. This has in this country excluded its 
use “for general street illumination, and restricted it largely to 
decorative lighting. 

To make the flame carbon arc long burning requires enclosing 
it similar as with the enclosed plain carbon arc to reduce the 
access of air. Since, however, by the consumption of the electrodes 
the luminescent materials contained therein escape as a smoke, 
means are required to deposit this smoke, by a circulating system, 
at some place where it does not obstruct the light by deposition 
on the globe. A number of such long-burning flame lamps have 
been designed, but none of them has yet found an extended in- 
dustrial introduction, probably largely due to conditions outside 
of the lamp mechanism: the yellow color of the light, the large 
unit of light, the expense of the electrodes, lack of steadiness, etc. - 

The only materials which thus far are used in flame carbon arcs 
as luminescent matter are calcium compounds, as fluorides, borates, 
phosphates, tungstates, etc. They give a very high efficiency, but 
a yellow light. White-flame carbons have not yet been introduced 
of an efficiency comparable with that of the more efficient yellow- 
flame carbons. | | 


144 ILLUMINATING ENGINEERING 


To some extent the flame carbon arc stands intermediate between 
the luminous are and the plain carbon are: the plain carbon arc 
gives light only by incandescence of the electrode terminals, the 
luminous arc only by luminescence of the arc stream, and the 
flame carbon are gives most of its light by luminescence of the arc 
stream, but also some light by incandescence of the positive carbon 
terminal. ) 

It is interesting to note, that thus far only three materials have 
been found which in the are give very high efficiencies of light 
production, reaching in large units values of 3 to 4 candles per 
watt; titanium, calclum and mercury. The first gives a white 
light, and is used in the magnetite arc; the second gives a yellow 
light, and is used in the flame carbon are; and the third is re- 
stricted to the vacuum arc. , 


28. The Mechanism of the Arc Lamp: Starting Device, Feeding 
Device, Steadying Device, Shunt Protective Device, Damping 
Device. Series Lamp, Shunt Lamp, Differential Lamp 


Due to the nature of the arc, as discussed above, all are lamps 
require an operating mechanism. 

Since the arc does not start spontaneously, a starting device is 
required. ‘l'his consists of a mechanism which brings the electrodes 
together, thereby closes the circuit between them, and then sepa- 
rates them and so starts an are. 

Since, in supplying the vapor conductor of the are stream, 
the electrodes consume, more or less rapidly, a feeding device is 
necessary, that is, a mechanism which gradually moves the elec- 
trodes together so as to maintain the proper length of the are 
stream. 

In constant-potential or multiple lamps a steadying device is 
necessary since, as seen, the are is unstable on constant potential. 
This consists of a resistance in direct current, of a reactance in 
alternating-current are lamps, which is connected in series to the 
arc, and usually made adjustable so as to accommodate the lamp 
to the different supply voltages met in electric-supply systems. 

In constant-current or series lamps a shunt protective device is 
necessary to close the circuit around the arc in case the circuit in 
the lamp opens by breakage or consumption of the electrodes. This 
usually consists of a shunt resistance, connected across the lamp 
terminals by a potential magnet. 


ELECTRIC ILLUMINANTS 145 


In addition thereto dashpots or other retarding devices are nec- 

essary to slow down the motion of the operating mechanism so as 
to draw the arc sufficiently slowly not to break, and to guard against 
over-reaching of the feeding mechanism. 
_ The operating mechanism is actuated by electromagnets or 
solenoids, frequently in combination with weights, and rarely 
springs, as the latter have usually proved unreliable in continuous 
operation. 

If only series magnets are used the lamp is called a series lamp; 
if only shunt magnets are used it is called a shunt lamp; if series 
and shunt magnets are used, a differential lamp. The series lamp 
regulates for constant current in the lamp, thus is not applicable 
where several lamps are connected in series; the shunt magnet 
regulates for constant voltage, irrespective of the current, and the 
differential lamp regulates for constant relation of current and 
voltage. The latter type is most commonly used. 

The different forms of arc-lamp mechanisms which are in in- 
dustrial use cannot be described here, but may be studied from the 
publications of the various arc-lamp manufacturers, which give 
detailed information, or by inspection of the exhibit of typical 
are lamps shown here,* and only some general principles can be 
discussed, which may enable a judgment of the correctness of 
individual operating mechanisms. 


29. The Effective Resistance of the Arc. Relation between Arc 
Length and Efficiency. The Short-Carbon Arc and the Long 
Luminous and Flame Arcs 


The effective resistance of the arc is not constant, but continu- 
ously and often rapidly varies or pulsates somewhat. The arc 
conductor is a vapor stream of a temperature very much higher 
than the surrounding air, and thus, even when well screened, more 
or less affected by air currents, drafts, etc. In the plain carbon 
are lamp, in which the heated terminals are the radiator, and the 
voltage consumed by the are stream is wasted, the arc length is ~ 
made as short as possible, without obstructing the light by the 
shadow of the electrodes, and the fluctuations of the are resistance 
therefore are moderate. In the flame arcs and luminous arcs, 
however, in which the light is given by the arc stream, and the 


* Also see “ Radiation, Light and Illumination,” p. 151. 


146 ILLUMINATING ENGINEERING 


potential drop at the terminals represents largely wasted power, 
efficiency requires a long are stream, and this is more sensitive 
to air currents, thus the fluctuations of the are resistance are 
greater, especially when very small currents are used, as necessary 
in smaller units of light. | 

The problem in arc-lamp design thus is to devise an operating 
mechanism which regulates as closely as possible for constant 
production of light by the arc lamp, and at the same time permits 
the use and economical operation of the are lamp on existing dis- 
tribution systems. 


30. Regulation of the Arc Lamp for Constant Inght Flux: the 
Floating System of Control of the Carbon Arc and tts Ad- 
vantages. Fixed Arc Length Required by the Luminous Are. 
Its Difficulties in Constant-Potential Lamps. The Compro- 
mise Control of the Flame Carbon Lamp 


In the plain carbon arc the light production depends on the 
current, but not on the are length, provided the latter is sufficient 
to minimize the shadow of the electrodes. Regulation for constant 
light flux, therefore, is closest by control for constancy of current. 
Thus the series magnet, which varies the are length to maintain 
constant current, is most satisfactory in constant-potential lamps, 
while in constant-current lamps the controlling mechanism merely 
has to maintain the are length sufficient for reducing the electrode 
shadows, and not too long to give too much waste of power. As 
the are length has no direct effect on the light, a floating system of 
control thus can be used, and is always used, as being easiest to 
operate. That is, one or both carbons are held floating by the 
counteracting forces of shunt and series magnet, or of series magnet 
and weight, and continuously move in adjusting the arc length to 
the fluctuations of are resistance. The voltage at the terminals of 
such a constant-current arc lamp thus shows very small fluctuations. 

Entirely different, however, are the conditions in the luminous 
arc. In this the light flux is proportional to the current and the 
are length, and any fluctuation of the are length, by a floating 
system of control, would give a corresponding fluctuation of light 
flux, and is therefore objectionable. A fluctuation of are resistance 
is accompanied by a change of luminosity, such that an increase 
of arc resistance and therefore of arc voltage at constant are length 
usually gives a decrease of luminosity, and thereby of light flux; 


ELECTRIC ILLUMINANTS 147 


and regulation for constant light flux therefore would require an 
increase of arc length at an increase of are voltage caused by an 
increased arc resistance. The floating system of control, by short- 
ening the arc at an increase of arc voltage, thus in this case controls 
in the wrong direction and accentuates fluctuations of light, and 
the nearest approach to proper regulation of light flux is given by 
maintaining constant arc length. Jn luminous are lamps there- 
fore always, as far as it is possible, a control for fixed are length 
is used. That is, the arc terminals are locked in position at a fixed. 
distance, and at intervals, depending on the rate of consumption, 
_ this distance is adjusted by resetting the arc. This fixed arc-length 
control gives a curve of terminal voltage, which fluctuates con- 
siderably, following the fluctuations of the are resistance. In con- 
stant-current circuits this is not objectionable, as the voltage fluc- 
tuations of the numerous lamps in series with each other superpose 
to a constant total voltage. In multiple or constant-potential lamps, 
however, the fluctuations of arc voltage may interfere with the 
operation, and thus either a very large inductance has to be used 
in series to the arc, to steady the current, or regulation for con- 
stant light flux more or less sacrificed by the use of a floating 
system of control, and as the result, the multiple-luminous arc 
lamp is less steady than the series arc lamp. 

In flame arc lamps, usually larger currents and thus longer arcs 
are employed, and a sluggish floating mechanism, if limited to 
work over a moderate range only, is less objectionable, but never- 
theless the light flux of the lamp is less steady than in the plain 
carbon lamp, and one of the main objections of the flame arc is its 
inferiority in steadiness of the light flux. 


$1. Classification of Arc Lamps. The Most Important Forms of 
Are Lamps - 


In classifying the different types of arc lamps we have: 

By the nature of the light production: the plain carbon arc, the 
flame carbon are and the luminous arc, the latter including the - 
mercury arc as vacuum arc. 

By the life of the electrodes: the short-burning arc and the long- 
burning are. The former giving a life of electrodes of from 8 to 20 
hours, depending on the current and the size of electrodes, the 
latter a life of 50 to 250 hours, or even much more, as with the 
mercury arc. 


148 ILLUMINATING ENGINEERING 


By the protection of the arc against the access of air: the open 
arc and the enclosed arc. 

By the nature of the supply circufts: the constant-potential or 
multiple arc lamp and the constant-current or series arc lamp. 

By the arrangement of the electrodes: the vertical arc and the 
horizontal are. In the former the electrodes are arranged vertically 
above each other, and the maximum light flux thus issues in the 
horizontal direction, except in the direct-current plain carbon arc, » 
in which the maximum light flux is downwards from the upper 
positive electrode as radiator. In the horizontal are the electrodes 
are converging downwards, and the maximum light flux thus is 
in the downward direction. 

The most important forms of are lamps thus are: 

The open plain carbon arc. A short-burning arc, anieh has 
survived in a few cities on 9.6 amperes series circuits. 

The enclosed plain carbon arc. Long burning, for multiple and 
for series circuits, on alternating and on direct current. The ma- 
jority of the arc lamps now in use are series alternating enclosed 
carbon arcs, on 6.6 amperes and on 7.5 amperes series circuits. 
This type of arc is, however, rapidly disappearing, due to its low 
efficiency. 

The intensified arc. It is an enclosed plain carbon arc, medium 
long burning, gaining its efficiency by the small size of the elec- 
trodes. It is mainly used for indoor lighting of high efficiency and 
white color, on constant-potential direct- and alternating-current 
circuit. 

The yellow-flame arc. Usually an open and short-burning arc, 
with converging carbons for downward distribution of light, used 
mainly for outdoor decorative lighting, and to some extent for 
second-class interior lighting. Its disadvantage is the yellow color 
of the light. 

The magnetite arc, mainly used on 4 amperes and 6.6 amperes 
direct-current series circuits, for street light, where it is taking the 
place of the series enclosed carbor arc. It is an open, long-burning 
are. 

The mercury arc or vacuum arc, mainly used for indoor lighting 
of high efficiency and steadiness. Its disadvantage is the green 
color of the light. 


ELECTRIC ILLUMINANTS 149 


32. Increase of the Efficiency of the Arc with Increasing Size of 
the Light Unit. Relation between the Efficiency of the Arc 
Lamp and the Current, Arc Length and Power, at Constant 
Are Length, Constant Current and Constant Power. The 
Conditions of Maximum Efficiency 


Unlike the incandescent lamp, in which the efficiency of hght 
production remains practically constant over a wide range of units 
of light, the efficiency of the arc lamp increases with increasing 
power consumption and thus increasing size of unit of light, but 


wi ce ae ; 
J00 7 10 
- Saeed oharonterites of Paras 
PL | | rhestenatite me |_| | 
pe RR BERS 
| 
Brak) | orpors [pal asi, (pela ANE 


ee 
| 
Sp ee Tay ay Dalal a ee Bee fe ee ieee 


Fie. 9. 


falls off with decreasing size of the hght unit, and the arc lamp 
thus is essentially a large unit of light, but for small units does 
not have the efficiency to compete with the modern incandescent 
lamps, while inversely for large units it reaches efficiencies of 
higher magnitude than possible with incandescent lamps. 

The relation of the efficiency of light production by the arc to 
the power consumption can, with fair approximation, be calculated, © 
especially for the luminous arc. 

For instance, in the series direct-current magnetite arc, the ap- 
proximate equation of the arc voltage is 


123 (1+.05 (1) 


150 ILLUMINATING ENGINEERING 


where the are length | is given in inches, and the approximate ex- 
pression of the light flux ®, in mean spherical candle-power, is: 


®=150h (2) 


(assuming, as approximately the case, the light flux as proportional 
to the are length and the current). 

For constant arc length | then follows, from equations (1) and 
(2), for different values of current i, the power consumption p=el 
and the efficiency y. Curves, for the arc length 1=.7 inches, are 
given in. Fig.-9, 


ae ieee oF 


AMM Gi 
Cae 





Fic. 10. 


For constant current, i=4 amperes, curves of the power con- 
sumption and of the efficiency for different are lengths are given 
in Fig. 10. 

As seen, with increasing current at constant arc length, and 
with increasing arc length at constant current, the efficiency in- 
creases, but the power consumption also increases. 

For constant power consumption, p=el, then follow, from equa- 
tions (1) and (2), values of arc length, are voltage and efficiency. 
They are plotted, for 300 watts and 500 watts power consumption 
in the arc, in Fig. 11 as function of the current. As seen, with 


Evectric ILLUMINANTS 151 


increasing current at constant power consumption, the efficiency 

increases to a maximum—which is higher with the 500-watt arc 

than with the 300-watt arc—and then decreases again. 
Determining then the condition of maximum efficiency, as func- 


reams see yy 
i SS eee 
oe eS SS 
Eee 
A bid edi td 


















pecaeeett 
i Se 
(7 i a 
RA 


ees 2 MLE 
DER PS 
Ts Si | 
PROSITE 
NES SSE 

NG SSR ee 
KS = RIA 
Baer are 










Fia. 11. 


tion of power consumption in the arc, gives the curves shown in 
Fig. 12. As seen, to operate at maximum efficiency, with increasing 
power consumption the current in the arc and the arc length has 
to be increased, while the arc voltage remains nearly constant. 
The efficiency rises rapidly with increasing power consumption. 


6 


152 ILLUMINATING ENGINEERING 


38. Comparison of the Arc Lamp and the Incandescent Lamp 


As seen from Fig. 12, the efficiency of the tungsten incandescent 
lamp, of approximately 0.66 candle-power per watt, is reached at 
70 watts power consumption. 















Ze an | 
CL Lo 





Fic. 12. 


Considering, however, that the efficiency is not the only factor 
in the cost, but that the cost of attention, trimming, etc., also en- 
ters, furthermore, that at the lower consumption some efficiency 
would have to be sacrificed to steadiness by increasing the current 
beyond, and therefore reducing the are length below that corre- 
sponding to maximum efficiency, the dividing line between tungsten 


ELEctTRIc ILLUMINANTS 153 


incandescent lamp and magnetite arc lamp for use in street lighting 
probably lies at about 100 to 150 watts power consumption, depend- 
ing on the individual conditions; below this the tungsten lamp 
above the magnetite arc is more efficient, other things being equal. 

Similar relations exist with other types of ares: with the flame 
carbon arcs, approximately the same relations would exist—except 
that the numerical values of efficiency are proportionally changed— 
provided that the size of the flame carbons is changed proportional 
to the current. If the same size of flame carbons is retained, the 
efficiency falls off more rapidly with the decrease of current, and 
_ increases more rapidly with its increase, due to the change of the 
rate of evaporation.. However, in economical comparison with the 
tungsten lamp, the very much higher cost of trimming, with the 
short-burning flame lamp, would probably shift the dividing line 
of economical use, between the tungsten lamp and the arc lamp, 
to higher values of power, while more efficient long-burning lumi- 
nous arcs would shift it to lower values of power. 


VACUUM ARCS 


84. The Low-Pressure Mercury Arc in the Glass Tube. The High- 
Pressure Mercury Arc in the Quartz Tube. Thetwr Charac- 
teristics 


The only industrially used vacuum arcs are the mercury arcs: 
the low-pressure mercury arc, operated in a glass tube, and the 
high-pressure mercury arc, operated in a quartz tube. 

In the mercury are the terminal drop is constant, and about 
13 volts, while the stream voltage is proportional to the are length 
and independent—within a certain range—of the current, but 
depends upon the diameter of the arc tube, and on the vapor pres- 
sure ; it increases with decreasing tube diameter and with increasing 
vapor pressure, so that in an arc tube of about 2 cm. diameter and 
a high vacuum it is as low as 0.5 volts per centimeter, and rises to 
8 to 10 volts per centimeter in a tube of 1 cm. diameter at a mer- - 
cury-vapor pressure about equal to atmospheric pressure. 

The mercury are is a luminous arc and stands at the one end of 
a series, of which the carbon arc stands at the other end; while the 
latter is the hottest arc the former is the coldest, and in the low- 
pressure mercury arc in a glass tube the temperature of the arc 
stream is only about 200° to 250° C. 


154 ILLUMINATING ENGINEERING 


Like all arcs, it requires a starting mechanism; the feeding is 
done by condensing the mercury vapor in a condensing chamber, 
and returning it to the negative electrode by gravity. 

A valuable characteristic of the mercury arc is, that it can be 
built of very good efficiency in smaller units than any other arc: 
as low as 80 to 100 watts. ) 

In the high vacuum of the mercury arc in the glass tube the 
arc length is very great at moderate voltages, and mercury arc 
tubes of over 3 feet length are operated on 110-volt circuits; in 
the quartz-tube arc, due to the high vapor pressure, the are length 
is short and comparable with that of other arcs of the same voltage ; 
an are length of 8 inches requires a 220-volt supply. 

Like all arcs, the mercury are requires a steadying resistance on 
constant-potential supply circuits. 

The light of the mercury are has the advantage of great steadi- 
ness and high efficiency, but the disadvantage of a green color, 
which is almost entirely deficient in red rays, and therefore greatly 
distorts colors. 


SENSITIVITY TO VARIATIONS OF THE ELECTRIC-POWER SUPPLY 


85. Comparison of the Various Forms of Incandescent Lamps and 
Arc Lamps Regarding Their Sensitivity to Variation of the 
EHlectric-Power Supply: 


The various forms of electric illuminants must find their place 
in existing electric distribution systems, either constant-potential 
or constant current. No electric circuit, however, maintains abso- 
lutely constant-potential respectively constant current, but fluctua- 
tions of greater or lesser extent occur, and it thus is of importance 
to know the sensitivity of the illuminants to variations of the sup- 
ply circuit. Since the limit of sensitivity of the human eye for 
changes of light flux is not much below 2 per cent, a sudden change 
of light flux of 5 per cent is not seriously objectionable, and a grad- 
ual change even of 20 per cent is hardly appreciable. The per- 
missible range of sudden and of gradual variation of the electric- 
supply system, and inversely, in a system of given regulation, the 
degree of satisfactoriness of an illuminant would then be deter- 
mined by the ratio of the change of light flux to the change of the 
electric supply causing it. 

In the following are given a number of approximate values of the 


ELECTRIC ILLUMINANTS 155 


percentage change of light flux of various electric illuminants, 
resulting from a change of the electric-supply voltage, current or 
power by 1 per cent. 

In the calculation of the incandescent lamp values, the curves 
of Fig. 2 have been used; the arc-lamp values are calculated from 
the characteristic curves of the arc, equations (1) and (2). They 
depend to a greater or less extent on arc length, per cent of steady- 
ing resistance, etc., and thus can be approximate only. 


APPROXIMATE VARIATION OF CANDLE-POWER, IN PER CENT 


For 1 per cent variation of— Power Voltage Current 
Incandescent lamps: 
Peemecmcarpon filament..........%.3...% 2.8 5.6 5.6 
PO TIMER ERTL OCR creole as oI Bieta Cac ale wig'S mies 2.5 4,45 Dar 
AIM BteNMIAINENt. . oo... ck ew awe ees 2.33 3.75 6.25 
Constant current arcs—75 volts per lamp: 
- Magnetite arc lamp ........ pote ote aaa As 1.42 ahr 1.0 
Flame carbon arc, differential control... 1.7 3.4 3.4 
Flame carbon arc, shunt control....... 1.55 +5, 1.55 


Constant potential arcs—110-volt supply, 33 
per cent steadying resistance: 


PAOPGUTYVOOTC 15 0:0.» A, Roe 5 erg 75 3.0 1.0 
Magnetite arc (constant are length).... 88 {Qe 1.0 
Flame carbon arc, differential control.. a ley f 3.4 3.4 
Flame carbon arc, shunt control....... 1.17 4.7 1.55 
Flame carbon arc, series control....... 2.65 2.65 


Incandescent lamps in general are much more sensitive to 
changes of supply than arc lamps, that is, require a closer regula- 
tion of the electric supply. 

Especially the arcs with constant fixed are length, as the mag- 
netite arc and the mercury are, are very little sensitive to changes 
of current, while the arc lamp with floating-feed and differential 
control is most sensitive to current changes, though less so than 
the incandescent lamp. 

Inversely, on constant-potential supply, the constant-pressure . 
are with fixed arc length shows the greatest sensitivity to voltage 
variations. This depends on the amount of steadying resistance, 
and decreases with increasing steadying resistance, while with less 
than 33 per cent steadying resistance the sensitivity increases so 
that the are soon becories inoperative. The least.sensitivity on 
multiple circuit is afforded by series control. 


156 ILLUMINATING ENGINEERING 


It thus would follow, that the incandescent lamps with high- 
positive temperature coefficient have an advantage on constant- 
potential supply, but a corresponding disadvantage on constant- 
current supply. On constant-current circuits the are lamps with 
fixed arc length, as the magnetite and mercury, would be most con- 
stant in their light production, and next thereto the lamps with 
shunt control, while inversely on constant-potential circuits these 
two operating mechanisms are most sensitive to voltage variations. 


ace) 
GAS AND OIL ILLUMINANTS 


By Arex. C. HUMPHREYS 


CONTENTS 


Introduction. 
Scope of lecture. 
Brief reference to the special character of the illuminants con- 
sidered. 
Petroleum and by-products—kerosene. 
Illuminants considered. 
Pintsch gas. 
Brief history. 
Present extent of use. 
How made. 
Externally heated retorts—low pressure. 
Internally heated generators, low pressure and high press- 
ure. 
Special characteristics. 
How employed. 
Lighting of railroad cars. 
Lighting of buoys, beacons, lightships, etc. 
Special appliances—especially pressure regulators, car lamps 
and buoy lanterns. 
Pintsch system provides for scientific distribution of light. 
Carburetted-air gas. 
Brief history. 
Present extent of use. 
Produced from certain hydrocarbons. 
Special characteristics. 
How employed. 
Isolated plants. 
Town plants. 
Special appliances. 
Acetylene. 
Brief history. 
Present extent of use. 
Carbide of calcium—CaC,. 
How produced from CaC,. 
Special characteristics. 
Liquefaction. 
Special precautions. 


158 ILLUMINATING ENGINEERING 


How employed. 

Isolated plants. 

Town plants. 

Portable lamps and lanterns. 
Special appliances. 


Introduction 


Those who unselfishly have taken the initiative in arranging for 
this course of lectures on the science and art of illuminating engi- 
neering, sparing neither time, thought nor effort, should be exempt 
from all unfriendly criticism. Then it should be understood that 
in 36 lectures, treating of 19 divisions of the subject, not more 
can be done, certainly with regard to some of the divisions and sub- 
divisions, than point the way to those who desire to devote them- 
selves seriously to the study and practice of illuminating engi- 
neering. 

This lecture is one of two which are expected to cover “ Gas and 
Oil Illuminants.” To Professor Whitaker has been assigned the 
open flame and the incandescent mantle, and to me Pintsch gas, 
carburetted-air gas and acetylene. It must be apparent that the 
hour and a half allotted to each lecture is entirely inadequate for 
a comprehensive consideration of the three systems named. 

It should be understood that this lecture, notwithstanding the 
sub-title of Part V, is not intended to cover coal gas, water gas 
or natural gas, which are the gas illuminants most generally dis- 
tributed and which, especially if taken together, furnish more 
artificial illumination than electric light together with the three 
illuminants here to be considered. 

As we proceed it will be seen that Pintsch gas, carburetted-air 
gas and acetylene do not compete with coal gas, water gas or 
natural gas, but are employed where these are not commercially 
available or obtainable, or where a special character of service is. 
required. 

Of these three sources of artificial illumination, two, namely, 
Pintsch gas and air gas, are made from oil. Pintsch gas is pro- 
duced by the destructive distillation of petroleum oil. It is not 
to be understood that the manufacture of oil gas is confined to this. 
process. Oil gas has been employed to a considerable extent in 
the United States and Europe for the illumination of small towns, 
factories, etc. Oil gas was so employed before it was applied in 


GAS AND OIL ILLUMINANTS 159 


compressed form to car lighting, and patents for oil-gas manu- 
facture were granted in the early part of the last century. 

In some few cases compressed oil gas has been employed for 
the lighting of small towns, the compressed gas being delivered 
in cylinders instead of through mains laid in the thoroughfares ; 
these undertakings were short-lived. In Scotland, prior to the 
production of petroleum in large quantities, a high candle-power 
gas was made from oil distilled from rich shales. 

By far the most extensive use of oil in the making of illumi- 
nating gas has been in the manufacture of carburetted water gas. 

Although the title of Division V includes oil as an illuminant, 
neither Professor Whitaker nor I are expected to consider it as 
a direct source of illumination. When we realize that refined 
kerosene oil is used throughout the whole civilized world, com- 
peting with all other sources of artificial illumination covered in 
these lectures and relied upon where these are not to be found, 
this well serves to illustrate the fact that the 36 lectures cannot 
be made to cover, even superficially, the whole field of artificial 
illumination. 

In this connection, let me refer you to “ Petroleum and Its 
Products,” by Sir Boverton Redwood, 2d Edition, 1906, two vol- 
umes, published by Charles Griffen & Company, London. This 
work is most valuable in itself, and also for the extensive bibli- 
ography annexed. 

Redwood gives an interesting and instructive history of the 
petroleum industry, beginning with a reference to an account writ- 
ten by Herodotus, 450 B.C., of a well producing “ asphalt, salt 
and oil.” Petroleum is now being produced in all parts of the 
world, and in many places in large quantities. Vast quantities 
have been discovered recently in Mexico, this oil being unusually 
rich in asphalt. 

The production of oil from coal and shale is of interest, especially 
as much of scientific and practical value was learned in the course 
of the evolution of the process and apparatus employed. 

I presume that other of the lectures included in this. course 
will give some account of the several by-products from the dis- 
tillation of petroleum which have been and still are employed in 
manufacturing water gas and enriching coal gas, and of the 
commercial utilization of these and other by-products of kerosene 


160 ILLUMINATING ENGINEERING 


manufacture which has enabled the great oil companies, especially 
the Standard, to produce kerosene at a minimum cost. 

The internal combustion engine, particularly as used in motor 
cars and motor boats, has, within the last few years, developed 
an extensive and rapidly growing market for the more volatile of 
the distillates which, together with the so-called gas oil, were 
almost a drug on-the market 25 years ago. 

Some idea of the magnitude and growth of the production of 
kerosene oil is found in the records for 1906 and 1909. In the 
former year the total production of kerosene is estimated by the 
Standard Oil Company to have been 48,000,000 barrels or 2,016,- 
000,000 gallons; and, in 1909, 53,000,000 barrels or 2,226,000,000 
gallons, an increase in 3 years of more than 10 per cent. 


Pintsch Gas 


Pintsch gas is so named after Julius Pintsch, of Germany, the 
founder of the great firm of that name. 

Pintsch gas is made by the destructive distillation of petroleum 
or other mineral oil in retorts (cast iron or clay) externally and 
continuously heated, or in generators filled with fire-brick checker- 
work, internally and intermittently heated. The product is in 
great measure a fixed gas, principally methane (CH,) and heavy 
hydrocarbons with a very small volume of hydrogen. The oil gas as 
so made, unlike water gas, is not diluted. 

The Pintsch system was originally developed for the lighting 
of, railway passenger cars. In the early days of railroading some 
trains were not run after dark, and in many cases where the trains 
were run through the night hours it was not considered necessary 
to furnish artificial illumination. The illuminants first employed 
were candles and oil lamps. 

In 1866 experiments were begun in Germany in the lighting of 
railway carriages with coal gas. It happened that in the United 
States the Reading Railroad also began to light some of its cars 
with coal gas in the same year. 

By reason of the limited space available on saattdaet cars for 
the storage of the illuminant, city gas was found to be too bulky, » 
and this suggested that the gas should be of comparatively high 
candle-power and be compressed into a greatly reduced volume. 
This led Pintsch to turn his attention to gas made from coal oil 
and petroleum. 


GAS AND OIL ILLUMINANTS TOL 


As compared with coal gas a double advantage was secured by 
the substitution of compressed oil gas for railroad lighting and 
similar service, for the oil gas, in addition to an initial illumi- 
nating power three or four times higher than that of coal gas, 
suffers a loss in illuminating power due to compression of only 
one-third to one-half of that of coal gas. This loss in compressing 
Pintsch gas to 10 atmospheres is only about 10 per cent. 

The advantages of compressed oil gas so markedly apparent in 
its application to the hghting of railway passenger cars were in 
even greater degree found to be applicable to the lighting of buoys, 
beacons, stake lights and lightships. In the late seventies Pintsch 
turned his serious attention to the development of a system to 
satisfy the varying demands of lighthouse authorities and met with 
prompt success. 

For the storage of compressed gas at the works Pintsch developed 
a process of welding by which were produced storage cylinders of 
large capacity free from seams or rivets. These seamless cylinders 
are now manufactured to a maximum size of 8 feet in diameter 
by 33 feet in length. For lighthouse work welded buoys were 
made of the several required shapes, the body of the buoy serving 
as a holder for the compressed gas. Difficult as was the welding 
of the storage cylinders, the welding of the buoy bodies was far 
more difficult. The application of this welding system to the manu- 
facture of buoys was particularly useful, because by eliminating 
riveted joints there was obtained the necessary strength and ca- 
pacity with the minimum of weight, and consequently the maxi- 
mum of buoyancy. | 

Pintsch also devised ’a wind- and wave-proof lantern which 
demonstrated its ability to maintain a steady and constant light 
under the severest weather conditions. 

In the use of compressed gas for car lighting, and still more for 
lighthouse service, it was necessary to develop a pressure regulator 
capable of receiving the gas at a pressure of from 150 pounds to. 
1 pound per square inch, and delivering it constantly to the burner 
supply pipe at such a reduced pressure as might be required for 
the most efficient operation of the particular burner employed. 
To meet this requirement Pintsch invented a regulator which, prac- 
tically without change, has met successfully all the requirements 
of nearly 40 years of the most varied and exacting service. 


162 ILLUMINATING ENGINEERING 


As far as I know, and I had a very personal experience with this 
regulator from the latter part of the year 1881 to the end of 1884, 
no gas, compressed or uncompressed, is supplied to the point of 
ignition under more uniform pressure than the gas supplied by the 
Pintsch system. J lay particular stress on this point because I 
know that questions have frequently been raised as to the com- 
plete reliability of such an instrument for constant and accurate 
regulation within narrow limits of outlet pressure. 

I will describe briefly a couple of tests which occurred under 
my own eye about the year 1883. The first was a test by the 
representatives of the United States Lighthouse Board of a Pintsch 
regulator and buoy lantern in competition with similar appliances 
of a rival system. The claim was made for the latter system that 
operating under 600 pounds pressure a decided advantage was 
secured by reason of the longer supply of light thus obtained from 
the one filling of the gas reservoir. Although the Pintsch goy- 
ernor was only tested and guaranteed for a pressure of 150 pounds, 
to meet the claims of the competitor, the Pintsch Company’s rep- 
resentatives offered to subject this governor to the 600 pounds 
pressure. Upon examination it was found that the storage holder 
of the rival concern was charged only to 300 pounds instead of 
600 pounds as claimed. U-water gauges were connected to the 
pipes connecting the governor outlets to the lanterns. The inlet 
pressures to both governors were first adjusted at 1 pound, and the 
corresponding outlet pressures as indicated by the U gauges were 
accurately observed and marked. By a quick movement of the 
hand the full pressure of 300 pounds was admitted to the inlet of 
each of the governors. In the case of the Pintsch governor the 
fluctuation of the governed pressure, as indicated by the U gauge, 
was found to be less than one-tenth of an inch of water and the 
flames were not affected; whereas in the other case the water was 
blown out of the U gauge and struck the ceiling of the room in 
which the test was being made, and the light was extinguished. 
In this test the lanterns were also subjected to conditions repre- 
senting a hurricane, the wind effect being obtained by the use of 
an air blower and the washing of the waves by. water delivered 
from a 2-inch hose under heavy pressure against all parts of the 
lanterns. The Pintsch lantern remained lighted while the other 
was extinguished. 


GAS AND OIL ILLUMINANTS 163 


The other case also served to show the reliability of the governor 
and the buoy lantern under extraordinarily severe conditions. Fol- 
lowing a heavy storm it was reported that one of the buoys recently 
anchored in New York Harbor had been extinguished. With the 
Lighthouse Board’s district inspector, I made a personal investi- 
gation. When we arrived at the buoy, from the tender it appeared 
that the light was extinguished. Determined that there should be 
no question as to the accuracy of the record I climbed into the 
cage surrounding the lantern of the buoy. Opening the lantern 
I found that the set-screw which regulates the size of the flames 
had been screwed down hard so that the amount of gas leaking by 
was only sufficient to produce flames practically non-luminous, 
with the result, even after the lantern was opened, that those on 
the lighthouse tender could not see the flames. That the record 
should not depend upon my word I demonstrated, by lighting a 
piece of paper at the flames, that the light was not extinguished. 
The delicacy of action of the governor and the efficiency of the 
lantern can be understood when I say that the flames were so 
small that after lighting the paper I game tiehed them by fanning 
them with a single motion of my hand. 

While the use of pressure regulators in connection with the dis- 
tribution of city gas introduces unnecessary complications, in the 
case of such special service as that which the Pintsch system has 
to perform, which necessarily demands special appliances designed 
and constructed to operate with mathematical accuracy, no addi- 
tional complication is introduced provided the regulator is com- 
pletely dependable. Given a gas delivered at a pressure well above 
that required for maximum efficiency with any illuminating burner, 
an important economic advantage is secured by the use of a gov- 
ernor which can be relied upon to reduce this excessive pressure 
to any desired point. This is well illustrated in the application 
of the Pintsch system to mantle lighting, as later to be explained. 

Between the years 1870 and 1880 the Pintsch system of lighting 
was introduced to a very considerable extent on the Prussian State 
lines. i 

Pintsch’s first United States parents were taken out between 
the years 1870 and 1880. 

In the year 1880 the Pintsch system was brought to the United 
States, being first applied in hghting the sound steamers of the 
Stonington Line and the cars of the connecting line of the New 


164 ILLUMINATING HNGINEERING 


York, Providence and Boston Railroad, now part of the New 
Haven system. The Pintsch plant for supplying the boats and 
cars was located at Stonington, Conn. . 

The next railroad to adopt the light was the Erie, the works 
for making and compressing the gas being built in the railroad’s 
yards in Jersey City. Shortly thereafter a similar plant was built 
at -Weehawken for the West Shore Railroad, and practically all 
of its passenger cars then being built were equipped for the new 
hght. 

At first the policy of the United States Pintsch Company was 
to induce each railroad adopting the system to own and operate 
its own gas works, one or more. This would have led to unneces- 
sary multiplication of gas works throughout the country. The 
policy was persisted in for a number of years, and in this is to be 
found the reason why the system made but little progress in the 
United States during the first years of the American company’s 
existence. It was not until a new element came into control of 
the United States Pintsch Company that this policy was aban- 
doned and more rapid progress made, the company undertaking 
the building of gas works and the supply of compressed oil gas 
to the railroads adopting the system. While now some of the 
railroads own and operate plants built for them by the company, 
the Pintsch Company owns and operates works of its own through- 
out the United States, Canada and Mexico, in many cases sup- 
plying several roads from the same plant. In a number of cities 
Pintsch gas is manufactured and distributed to the railroads by 
the local gas company operating in partnership with the Pintsch 
Company and the railroads served. 

The Pintsch system is in use practically throughout the civilized 
world. Up to date about 180,000 cars in all are equipped for 
Pintsch light. 

Up to April 30, 1909, there were in service in the following coun- 
tries, namely, Great Britain, Germany, Holland, Belgium, France, 
Portugal, Denmark, Russia, Tunis, Sweden, Austria, Italy, United 
States, Brazil, Argentine Republic, Uruguay, Egypt, India, South 
Africa, Canada, Australia, New Zealand, Algiers, Spain, Japan and 
China, buoys, 1947; beacons and stake lights, 485; lightships, 96; 
these being supplied from 77 charging stations. 

A later return, covering the lighthouse service for the United 
States and Canada, shows that on August 10, 1910, the number 


/ 


GAS AND O1L ILLUMINANTS 165 


of buoys in service in these two countries was 461, an increase of 
30 in the 15 months. 

Up to June 1, 1910, there were in the United States, Canada 
and Mexico 93 Pintsch gas works, supplying compressed gas to 
360 railway stations. In the same territory, up to January 1, 
1910, the number of cars equipped for the Pintsch system was 
35,137. ) 

During the last few years the Pintsch system has been further 
developed to secure the additional advantages to be obtained 
through the use of incandescent mantle burners. Up to date there 
are mantle lamps installed in railway cars as follows: France, about 
95,000; Great Britain, about 61,000; other European countries, 
about 152,000; United States, Canada and Mexico, about 80,000; 
total, about 388,000. These figures represent mantle-lamp equip- 
ment for about 55,400 cars. 

Pintsch gas, as has been stated, is obtained by the destructive 
distillation of oil. In the early days of the system oil produced 
by the distillation of coal or shale was used. Of late years crude 
petroleum oil and its distillates have been employed, market con- 
ditions controlling the choice. The crude oil can be satisfactorily 
employed and was at one time largely used. To-day market con- 
ditions generally lead to the use of a distillate. 

At first, and until recent years, the gas was manufactured only 
in cast-iron retorts externally heated. Much of the gas is still 
so made. ‘Two retorts are set in each “bench” or furnace, the 
two retorts being so connected at their back ends that the gas passes 
from one to the other. The oil is introduced at the front of the 
upper retort and falls upon a removable sheet-iron tray which col- 
lects most of the carbonized oil. The gas and vapor produced in 
the upper retort pass down to the back of the lower retort, and so 
through to the front of the bench, passing by a decension pipe to 
the hydraulic main located below the floor of the house. Issuing 
from the hydraulic main the gas and vapor pass through a dry 
scrubber, condenser, purifiers and station meter, and are collected 
in the low-pressure storage holder. From the storage holder the 
gas is drawn by a compressor, compressed into one or more of 
the welded cylinders before described, and is then ready for dis- 
tribution through the high-pressure pipes to the cars or transport 
holders. All necessary precautions are taken to trap the liquid 
hydrocarbon thrown down by the process of compression, the object 


166 ILLUMINATING ENGINEERING 


being to obtain a thoroughly dry gas, which result is secured to a 
remarkable degree. 

The early German practice limited the compression between 8 
and 10 atmospheres. The more recent practice, especially in the 
United States, is between 12 and 14 atmospheres. 

Particularly in connection with the larger plants, clay retorts, 
as used in coal-gas manufacture, came into use. This change, by 
reason of the porosity of the clay retorts, has made necessary the 
employment of exhausters to draw the gas from the retorts and 
push it on through the other parts of the plant to the storage 
holder. 

When the gas is distilled in clay retorts the distillation is com- 
pleted in a single retort, the oil being introduced through a 
wrought-iron pipe carried through the front of the retort, extending 
nearly its entire length and open at the end. The oil, gas and 
vapor issue from the open end of the pipe and return through 
the retort to the front. The gas and vapor issue from the front 
of the retort and pass by an ascension pipe to the hydraulic main 
located on the top of the bench, and from there, as before described, 
to the storage holder. 

Some years ago experiments were indents to determine if 
greater economy could be secured by distilling the oil in generators 
internally fired. This is necessarily an intermittent process and 
so is markedly differentiated from the continuous retort process. 

The generator consists of a steel shell 6 feet in diameter and 
about 12 feet in height. It is lined with fire-brick and the interior 
is divided into two compartments, a smaller lower compartment 
_ which serves as a combustion chamber, and a larger upper com- 
partment which is filled with fire-brick checker-work nearly up to 
the top of the shell; this upper chamber terminates in a cone, upon 
the top of which is a stack valve. 

The tar, obtained as a by-product from the distillation, is used 
as fuel. This is injected into the combustion chamber below the 
checker-work by means of a liquid-fuel burner. A mechanical 
blower produces the necessary forced draft. 

As soon as the generator has been “ blown ” to its proper working 
temperature the tar fuel, steam and air are shut off and the stack 
valve is closed. Gas oil under pressure is then injected through 
three oil nozzles located in the top of the generator and the finely 
divided oil is thrown upon the checker brick. The oil vapor so 


Gas AND OIL ILLUMINANTS 167 


formed passes down through the heated checker bricks and is so 

decomposed, the gas produced finally issuing from the generator 
through the take-off pipe located in the side of the combustion 
chamber. 

The cycle is divided into a heating period (“blow”) of about 
5 minutes, and a See period (+ 7un ~~) tof from. 6 -to-8 
minutes. 

The rate of flow of oil is regulated by the so-called trowel test 
which the gas maker applies at short intervals. This test con- 
sists in permitting a fine jet of the hot gas to impinge upon the 
polished blade of a mason’s trowel, the figure made upon the trowel 
by the condensed tar indicating to the practiced eye the amount 
of condensable vapor in the gas. With care in operation the gas 
is obtained of quite uniform quality in spite of the gradually de- 
creasing temperature of the generator. 

About 1500 feet of the gas are made per “ run.” 

The gas after leaving the generator is dry scrubbed and cooled, 
and is then collected in a “ relief” holder. From the holder it is 
drawn by the compressor through the purifiers and station meter, 
and then compressed into the high-pressure storage holders at a 
pressure of about 14 atmospheres. 

It is found that by this method of intermittent distillation in 
internally fired generators a gas can be obtained about 10 per cent 
higher in candle-power than by the retort process, with the at- 
tendant advantages of largely reduced floor space, reduced cost of 
construction, and lower manufacturing cost due to economy in 
fuel, labor and repairs. 

In order to simplify the apparatus and reduce the investment at 
stations where the output is small, a still later development is the 
generation of the gas under the pressure required for delivery. 
(See Fig. 1.) 

To withstand this heavy pressure, the generator shell is con- 
structed of heavy steel plates. ‘The shell is divided as follows: 
At the bottom, a combustion chamber; above, a chamber filled with. 
fire-brick checker-work ; above this, a space for the oil sprays; and 
above this, another chamber filled with checker-work. 

The “blow ” and “run” occur as in the low-pressure generator. 
In order, however, to check the rapid decomposition of the oil, 
which would otherwise occur when operating under heavy pressure, 
steam is injected with the oil into the generator. The steam so 


168 ILLUMINATING ENGINEERING 





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Fic. 1.—Pintsch Gas High-Pressure Generator Plant. 


GAS AND O1L ILLUMINANTS 






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Fic. la.—Pintsch Gas High-Pressure Generator Plant. 


169 


170 ILLUMINATING ENGINEERING 


used acts as a carrier and protector for the oil vapor and gas, and 
does not react with the carbon of the oil to produce water gas, the 
temperature of the generator being too low for this reaction. 

The steam enters the generator at the top, being superheated in 
passing down through the upper checker-work. Coming to the 














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Fig. 1b.—Pintsch Gas High-Pressure Generator Plant. 


intermediate chamber it meets and mingles with the finely divided 
oil, and then steam and oil vapor pass down through the second 
checker-work wherein the oil vapor is decomposed. The gas and 
superheated steam finally leave the bottom of the generator through 
the take-off pipe in the side of the combustion chamber. The gas 


Gas AND O1L ILLUMINANTS bal 


and highly superheated steam are then dry scrubbed and cooled, 
and the gas, tar and water are charged into the first storeholder 
under a pressure of about 14 atmospheres. Passing from the first 
storeholder the gas is purified under pressure and is then stored 
in other high-pressure storeholders. 

Before the next “blow” the gas and oil vapor which remain in 
the generator under pressure are displaced by means of steam at 
sufficient pressure, being thus forced through the scrubbing and 
cooling devices: and into the storeholders. 

_ This high-pressure plant is more simple and compact because 
the low-pressure gas holder and compressing apparatus are not 
required. 

Three of these high-pressure generator installations have been 
put into operation, and one of these has been operating satisfac- 
torily for 2 years; three more are now in process of construction. 

In both low- and high-pressure systems the generators are oper- 
ated at a temperature of about 1200° F. 

The average of analyses of 25 samples of compressed Pintsch 
gas was as follows, and furnishes a representative indication of its 
composition : 





NR EA Eee ee cages w suone oo sigtore''s «evn 8 60% 
Heavy illuminants: 
Benzene C,H, ..... ae 
POD VEC NC dO allen ep) ks cusdicioap ih od of diets Ole Salart 35 
Ethylene C,H,, ete. 
Ob erect: OE NR RT ars ais a ary ate ve: 5 
nee NN rr es Ca cea hs he te o athe ters 4.5 
100.0 


Specific gravity .80 to .85. 

Ignition temperature, determined by Milton L. Hersey, chemist and 
chief engineer of tests of Canadian Pacific Railway, made at McGill 
University 1562° F. or 850° C. 

Explosive limit between about 4 per cent and i0 per cent of the gas. 


The horizontal candle-power of the compressed gas, tested in open. 
flat-flame burner sufficiently small to avoid smoking and calculated 
to the 5 feet per hour consumption, is about 40. By reason of the 
necessarily small rate of consumption this does not furnish a re- 
liable indication of the candle-power. The spherical illuminating 
power of the lamps, naked flame and mantle, as later to be stated, 
are the values to be considered for purposes of comparison. 


172 ILLUMINATING ENGINEERING 


Most of the Pintsch gas is used for the lighting of railroad cars. 
While a relatively small amount is used in lighting buoys, beacons, 
etc., the service performed is one of commanding importance. In 
the early days steamers and ferry boats were satisfactorily lighted 
by this system. Many of the ferry boats plying in New York 
Harbor were at one time lighted by coal gas, uncompressed or com- 
pressed. In some cases these methods were superseded by the 
Pintsch system. The advance in the art of electric lighting, coupled 
with the special adaptability of electric lighting to the illumina- 
tion of vessels equipped for steam power, led naturally to the re- 
placement of compressed gas by the electric light. 









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Fig. 2.—Pintsch Gas Regulator. 


The Pintsch regulator deserves more than a passing notice. (See 
Fig. 2.) The essential parts of the regulator are a needle valve 
of special form and a large diaphragm made of leather so treated 
as to be gas proof and extremely flexible. The diaphragm is sub- 
jected only to the reduced or regulated pressure and controls the 
movement of the valve through a lever of such proportions that the 
pressure of the valve against its seat is 11 times the total pressure 
against the diaphragm. A pair of springs acting on the lever 
through a knife edge oppose the pull of the diaphragm and can 
be regulated so as to give the required outlet pressure. The needle 
valve, so controlled, is relied upon to exclude the high pressure 
from the interior of the regulator, no auxiliary valve being em- 
ployed for that purpose when the lamps are shut off. 

For the illumination of railroad cars many forms of naked-flame 
lamps have been employed. These have all been designed to meet 


~ 


GAS AND OIL ILLUMINANTS 173 


the exacting requirements necessarily involved in the lighting of 
cars running at varying speeds, subject to abrupt stops, so ven- 
tilated that the lamps are required to resist strong air draughts, 
and under the care of trainmen who cannot be relied upon to give 
the lamps expert attention. 

As this is one of a number of lectures on illuminating engi- 
neering it is in order that I should call particular attention 
to the fact that the engineers of the Pintsch Companies here 
and abroad have recognized constantly that they were required to 
solve their problems from the standpoint of the engineer of il- 
lumination. Not only has the effort been to secure the greatest 
amount of hght from a minimum of material and at a minimum 
cost, but the effort has been to distribute this light so as best to 
serve the travelling public. It has always been recognized that an 
important element in the problem was to secure ‘an effect which 
would be pleasant and restful to the eye. All the problems in- 
volved have been under discussion and subject to experimentation 
constantly. It was recognized that the first step was to obtain a 
steady flame, free from flicker, and that this must be secured 
through the design of a draught-proof lamp and a pressure regu- 
lator at once sensitive and reliable. 

I know that some hold that illuminating engineering was not 
the subject of scientific study by gas engineers until the electric 
light engineers led the way. I am inclined to think that some 
of our electric light associates in the I]luminating Engineering 
Society are of this number. Many facts in regard to gas engineer- 
ing practice could be cited against this proposition. In addition 
to the record made by the Pintsch engineers, let me refer to one 
example, which is notable in this connection. Some few years ago, 
at a meeting of a committee of our Society, I learned that the 
electrical engineers present were of the opinion that a notable 
advance in the science of illumination was made when rooms were 
first illuminated by light reflected from sources hidden from the 
eye, and that this advance was to be credited to the electric light _ 
engineers. I then described the lighting of the Liverpool Phil- 
harmonic Hall by naked gas flames placed so as to be hidden from 
view by the plaster cornice, the light being reflected down into the 
hall from the curved surface of the ceiling. This installation was 
made 50 years before I first saw it, which was over 10 years ago. | 

I trust I may be pardoned for this little digression, and especially 
by my electric light associates. 


174 ILLUMINATING ENGINEERING 


Figure 3 shows a flat-flame four-burner railroad car lamp. It 
is here to be borne in mind that the methods of hanging and the 
design of the body of the lamp have been varied to meet practical 
conditions and the demands, sometimes artistic and sometimes not, 
of the railroads’ managers. In this lamp the air supply to the 
burners passes through the upper portion of the body and so 
into the cylinder enclosing the four chimneys, down into the lower 
portion of the lamp and so into the globe, where it reaches the 
flames. The products’ of combustion go up past the central re- 











£ at —. 


268 


























Fie. 3.—Four-Burner Flat-Flame Railroad Car Lamp 


flector, and so on up through the chimneys, some of the sensible 
heat of the products of combustion being transferred to the in- 
coming air. | 

The four burners togéther consume about 314 feet of gas an 
hour, and give 30 to 35 mean hemispherical (lower) candle-power. 

Of recent years the Pintsch Companies have devoted much at- 
tention to the application of incandescent mantles to car lighting 
and buoy lighting. Experiments with vertical mantles were not 
successful, by reason of frequent breakages. After the trial of 
many devices to reduce the effect of shock the engineers of the 
United States Company solved the problem by means of a strong 
inverted mantle rigidly fixed to the burner. To secure increased 


GAS AND OIL ILLUMINANTS 175 


strength these mantles are made heavier than the ordinary mantle, 
and to compensate for the loss in illuminating power due to this 
increase in mass the gas is supplied to the burners at a pressure 
of 2 pounds. This advantage is secured by the use of a compressed 
gas controlled by a reliable governor. It is a rather remarkable 
fact that the lamps are not provided with means of adjustment. 
The gas orifices and air inlets are drilled to standard sizes, and, 





Fig. 4.—Single Mantle Car Lamp. 


having passed the calibration tests, the lamps are erected .as turned 
out from the factory. 

These mantle burners consume 2 feet of gas an hour and give - 
(the mantles alone) 90 to 100 horizontal candle-power without the 
aid of reflectors. As arranged in the car lamp, they give a mean 
hemispherical candle-power of 90 to 100. Comparing with the 
flat-flame lamps already described, the lighting effect is about 4 
to 1, and with the same gas storage capacity the length of period 
between fillings is practically increased 60 per cent. 


176 ILLUMINATING ENGINEERING 


These inverted mantles as now used have established a satis- 
factory life record. Some little time ago a careful observation was 
made of their service on 25 steam railway cars engaged in New - 
York suburban traffic. These cars were equipped with 125 lamps. 
The cars were handled in the regular way by the trainmen, who 
were not informed that the lamps were under special observation. 
The Pintsch employees, however, renewed all broken mantles so 
that an accurate record of the mantles used might be obtained. 
The result of this test for the 125 lamps was an average mantle 


GAS TANK 


PRESSURE REGULATOR NO 254 
FILLING VALVE NO ©344 





Fic. 5.—Car Equipment for Pintsch Lighting. 


life of 376 days. This shows a notable improvement even over 
the old inverted mantle as first made. 

The construction of the lamp is shown in Fig. 4. The regu- 
lated gas at 2 pounds pressure is admitted through fitting No. 3146, 
and passes down to a strainer of peculiar construction placed in the 
vertical channel. The gas issuing therefrom is met by the air 
pulled in at the sides by the gas, and the gas and air mixture then 
passes down unobstructed to the burner, which consists of a metal 
dise accurately drilled with seven orifices. 

Fig. 5 shows car equipment for Pintsch lighting; Fig. 6 is an 
interior view of a railway coach lighted with mantle lamps, and 
Fig. 7 is an illumination diagram for such a coach. 

I cannot conclude this section of my lecture without describing, 
at least briefly, the Pintsch buoy, a very beautiful example of 


GAS AND OIL ILLUMINANTS barge 


specialized engineering akin to illuminating engineering. (See 
Fig. 8.) The buoy body is a seamless welded-steel shell designed 
and constructed to withstand the high pressure of the gas stored 
therein, and to afford ample buoyancy for the support of the anchor 
chain, lantern and other parts. The buoy bodies are made in 





Fic. 6.—Interior View of Coach with Mantle Lamps. 


different shapes to meet varying conditions as to depth of water, 
anchorage, tideways, etc. A suitable tower surrounded by a cage 
supports and protects the lantern and carries a platform to afford 
a footing for the attendant when lighting or adjusting the flames. 
The lantern is designed and constructed to protect the light from 


178 ILLUMINATING ENGINEERING 


rain, waves and wind under the severest possible conditions to be 
found close to the surface of the sea. The base of the lantern 
forms the case for the pressure governor. 

In the original lantern the burner was placed inside a Fresnel 
dioptric fixed-light lens which, by bending the light rays, confined 
them approximately between two horizontal planes, thus increasing 


as wn Ww > | 


LAMP No.3508. 
BOWL FILE 272 























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EXTREMEVARIATION — _2-15. __ _ __- 


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s ie | | NoOFLAMPS.. — 2 gee eee 
RTT | pube s-ce No.OF BURNERS — 1c et cee 

HN i 5 10 = GLASSWARE-GROUND BOWL —OPAL TOP 
Pry Se uzes | GAS PER HOUR: = 2559:04 2 es 
HIGH WS PRESSURE ______. @ POUNOS___ __ 
_ DAS UC Gr TEMPERATURE 222 ee tee 

4 | TTT | esa FREEGASPERHOUR ___— 9.96 
NL ate AVERAGE FOOT CANDLES #03 __ _ __ 
TAN HHHHh 7-15 16 aes MAX.FOOT CANDLES _ __ 3-40 _ __ ___ 
hi ii 191 ap) MIN. 2233: aie coh sc See 


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156 138 CUBICFEET GAS PER HOUR FOR 
: [3 24] |: Sa 1FT.CANDLE ILLUMINATION “+90 _ _ 

224 340 D 

25 26 

240 3.05) 

27 28 

194 218 

29 30 

184 220 





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Fig. 7.—Illumination Diagram for Coach with Mantle Lamps. 


the power and range of the light. Some of the lanterns thus 
equipped are still in use. In order to give the light a specific char- 
acteristic and at the same time reduce the gas consumption and so 
increase the interval between gas chargings, a further improvement 
was made by the addition of a flashing mechanism. This is a 
simple and reliable device which controls the flow of gas to the 
burner so that the gas is ignited and extinguished at intervals, the 


GAs AND OIL ILLUMINANTS 179 


lengths of which are predetermined to meet the particular condi- 
tions of each case. This automatic mechanism is enclosed in a 
chamber located immediately above the governor, and is actuated 
by the gas flowing through this chamber on its way from the 
governor to the flash-light burner, which is ignited by a pilot light 
burning continuously and receiving its gas supply direct from the 
governor. 








\ 


| 


\ 


eee cees 
ee ee eeeee 
see ereeesl 


Fig. 8.—Pintsch Buoy. 


The relative periodicity of light and darkness can be varied by 
the adjustment of the mechanism to meet varying requirements. - 
The standard adjustment gives periods of equal lengths, usually 
5 seconds or 10 seconds each. If desired the periods can be made 
non-uniform. 

The latest form of this mechanism provides for the buoy being 
used either as a fixed or flash light, as required for any location; 
all the buoys as now built and supplied are so equipped. Nearly 


180 ILLUMINATING ENGINEERING 


all the buoys now in use are equipped with the flash-light mechan- 
ism, and most of these are of the convertible type. 

As the fixed-light lens permits the rays to radiate horizontally 
through 360 degrees, to still further increase the power and range 
of the buoy lights a “ bull’s-eye” or flash lens can be employed 
instead of the flashing mechanism just described. If desired, a 
series of these lenses can be grouped in a circle around the light 
source. That the light may be visible at all points in the horizon 
the bull’s-eve lens, or series of lenses, must be revolved. This is 
effected by a motor driven by the gas flowing to the burner. This 
lens arrangement delivers a light at least 20 times as powerful as 
that from the fixed-light lens. 

An additional advantage is that the characteristic of the buoy 
light can be further determined by the design and the relative 
positions of the lenses of the series. There are comparatively few 
of the revolving lenses in service. 

Until recently flat-flame burners were used exclusively in the 
Pintsch buoys, but mantle burners are now displacing the flat 
flames. The older lanterns are being remodeled for mantle burners, 
and all new lanterns are of this type. (See Fig. 9.) 

As compared with the flat flame the mantle burner gives a 
candle-power three times as great, and its intrinsic brilliancy is 
ten times as great, resulting in greatly increased power for the 
same consumption of gas. The flat-flame burners are made for 
different rates of consumption, while the mantle burners are made 
for one rate only. 

Bells operating either above or below the surface of ne water 
and actuated by the flow of gas supplying the burner are in some 
cases attached to these buoys. 

With one gas charge these buoys will run from 55 to 528 days; 
the size of the buoy body, whether flat-flame or mantle burner, 
whether fixed or flash light, and if flat flame, the size of burners, 
determining the number of days. 

Stationary beacons and light ships are also ‘eantDee for and 
operated with Pintsch gas. 

Gas under a pressure of 100 atmospheres is now being used ex- 
tensively for this marine work. For beacons and light ships it 
is burned direct from the cylinders in which the gas is conveyed. 
In the case of buoys the high-pressure cylinders obviate the neces- 
sity for large storage holders and compressors on the supply tender, 


GAs AND O1L ILLUMINANTS 18] 





m-Outer Cap 


Lantern 
Glass 





H. «, Lers—} 
hi Hantle RY 
ohelete A 
Mo. 5664 









flashing Chamber 


Seale 671" 





Fie. 9.—Pintsch Buoy Lantern with Mantle Burner. 


182 ILLUMINATING ENGINEERING 


the buoys being charged direct up to 10 atmospheres from the 100- 
atmosphere cylinders. 

It is found that about 133 volumes of the gas can be stored under 
a pressure of 100 atmospheres, and that little or no additional loss 
in candle-power is suffered in carrying the compression from 14 to 
100 atmospheres. There is an additional deposit of liquid hydro- 
carbon, as indicated by the increased storage volume, but if the 
outlet pipe is sealed in this liquid the liquid revaporizes, and at 
the reduced pressure of 14 atmospheres and below it is carried 
through the appliances to the burner practically as a dry gas. 

Let me conclude by pointing out two features of the Pintsch 
system of great practical advantage to its patrons. 

In connection with the filling of the cars it is important that the 
amount of gas delivered to each car should be readily ascertainable 
for record. It is even more important that the attendant should 
be able to tell by inspection at any time how many hours of lighting 
are provided for by the gas in the cylinder. Both of these re- 
quirements are met by making the cylinders of standard sizes, the 
cubical contents in feet being marked and recorded. A high-pres- 
sure gauge showing the pressure in atmospheres is attached at 
each car-filling valve. The simple calculation of multiplying the 
gauge reading by the capacity of the cylinder gives the available 
volume of gas contained. 

Another important feature is that throughout the territory cov- 
ered by the United States Pintsch Company all parts of machinery 
and all fittings are interchangeable. The design of the smallest 
and apparently most insignificant part has been carefully con- 
sidered. The engineers have from the first recognized that they 
were offering to perform a special service involving many diffi- 
culties. As a result, a system has been developed that provides for 
the supplying of Pintsch gas to any railroad car equipped with 
Pintsch standardized appliances, no matter how far that car may 
be from its home territory, provided it is within reach of any one 
of the 93 gas works or any one of the 360 Pintsch gas-supplied 
railway stations located in the United States, Canada or Mexico. 


Carburetted-Air Gas 


Carburetted-air gas consists of atmospheric air to which hydro- 
carbon vapor has been added, the proportions of air and vapor vary- 
ing with the process employed. 


Gas AND OIL ILLUMINANTS 183 


The application of carburetted-air gas as an illuminating and 
heating agent to meet certain special conditions has been an in- 
dustry for about 40 years. As a source of energy in the internal 
combustion engine its use has been of late greatly increased and 
extended. 

Carburetted-air gas machines can be grouped in two classes, those 
operated without heat and those operated with heat. Those of the 
former group have been more generally employed, especially where 
the principal service has been lighting. In operating with cold 
air it is necessary to use refined highly volatile gasoline; but if 
steam or other heat source is employed to assist evaporation, the 
somewhat less volatile and less expensive naphthas are used. 

Carburetted-air gas differs fundamentally from coal gas, water 
gas or oil gas through the fact that whereas in the process of mixing 
the liquid hydrocarbon is vaporized it is not changed chemically, 
while in the case of the other three gases the manufacturing process 
to which the coal or oil is subjected converts the hydrocarbon into 
fixed gases in major proportion and certain vapors in minor 
proportion. 

In the distillation of crude petroleum, as the temperature of the 
still rises, the several distillates are driven off successively accord- 
ing to the following approximate classification : 


Readings of Corresponding 


He ainctse Specific Gravity Trade classification 
90° and above 0.6363 and below Rhigolene & cymogene 
90° to 80° 0.6363 to 0.6667 Gasoline 
80° to 70° 0.6667 to 0.7000 Light naphtha 
i tO our 0.7000 to 0.7368 Heavy naphtha 


Following these distillates come the kerosenes, lubricating oils, gas 
oil, solid hydrocarbons, tars and solid carbons or hydrocarbons. 
While refined gasoline of 90° B. (sp. gr., .6363) is obtainable in 
this country, the price and the extra difficulty in holding it against 
evaporation have operated to prevent the development of a wide 
market for this grade. , 
Refined gasoline lighter than 86° B. (sp. gr., .6481) 1s not gen- 
erally obtainable in this country. This distillate consists mainly 
of hexane (C,H,,) and pentane (C,;H,,), with some still lighter 
and some heavier hydrocarbons. It should evaporate under condi- 
tions of use without giving off at first an excess volume of light 
vapors or leaving unvaporized heavy residues. A distillate capable 


184 ILLUMINATING ENGINEERING 


of meeting these conditions can be obtained only by repeated dis- 
tillations in the refinery to isolate in the gasoline those closely 
related hydrocarbons which will evaporate in approximately the 
same volumes under the same conditions. The refining process 
must also provide for the removal of all traces of tar which other- 
wise would deposit in the smaller pipes, gum the floats and clog 
the burners. For the making of air gas the more general practice 
has been to use a gasoline of about 84° B. (sp. gr., .6542). While 
this distillate leaves unvaporized a little residue, the amount is 
small, and as a rule does not have to be pumped out oftener than 
every 6 to 12 months. It is interesting to note that the residue 
is about 63° B. (sp. gr., .7254). The nomenclature which developed 
to identify the distillates of petroleum in many cases is based only 
upon a commercial or industrial suggestion. As the names given 
to several of these distillates have been the occasion for considerable 
confusion, a few words of explanation may not be out of place. 
When these lighter distillates from petroleum were first obtained 
uses for them in the arts were still to be found. In manufacturing 
kerosene, for which there was a ready market, the refiners were 
embarrassed to find storage for these distillates produced as by- 
products, and for which there was little or no market. At that 
time benzene—a hydrocarbon having the chemical formula C,H,, 
obtained principally ‘from the distillations of coal-tar—possessed 
a considerable value in the industries as a solvent for fats and 
greases and an enricher for gas. It soon became clear that some 
of the lighter distillates of petroleum could be used as a substitute 
in part for benzene, and thus a commercial reason was furnished 
for designating these distillates by the name benzine. In the same 
way other distillates of coal-tar, known as light and heavy naphthas, 
had their names pre-empted for other petroleum distillates. As the — 
nomenclature thus developed fails to meet the requirements of a 
technical terminology the result naturally has been a most em- 
barrassing confusion in technical and industrial literature. As an 
example, there are uses for benzene and coal-tar naphthas for which 
the petroleum distillates cannot be substituted; hence the need to 
be sure whether the substance under consideration is benzine or 
benzene in the first case, or naphtha or petroleum “ naphtha” in 
the second case. Another feature of commercial practice which 
has led to confusion is that of designating the specific gravity of 
petroleum distillates by the Baumé hydrometer readings, even to 


GAS AND OIL ILLUMINANTS 185 


the extent in some cases of calling that reading the specific gravity. 
This is all the more unfortunate for the reason that in the upper 
part of the scale as the Baumé reading increases the distillate is 
of a lighter specific gravity, and in the lower part of the scale as 
the Baumé reading decreases the distillate is of a heavier specific 
gravity, the Baumé reading of 70° indicating a specific gravity 
of .70. 

An additional complication arises from the fact that the Baumé 
scale for liquids lighter than water is calculated on more than one 
formula, and therefore the tables used in converting Baumé degrees 
to specific gravity do not always agree. The values here given are 

. 140 
130+ Baumé reading 
which is the American standard. Another formula more often fol- 

146.3 
146.3-+ Baumé reading” 
the tables are frequently given without the formula and the unwary 
may be deceived. Some authorities are careful to state in the title 
of the table, “ American Standard.” In the majority of books of 
reference the tables do not go above 80° B., and in some the tables 
are even more limited. Jor these reasons for American prac- 
tice. it is convenient to remember the formula sp. gr. equals 

140 
130+ Baumé reading ° 

The volume of gasoline vapor that can be carried by a given 
quantity of air depends upon the temperature, the pressure remain- 
ing constant. The ability of air to take up and hold in suspension 
gasoline vapors increases very rapidly with the increase in tempera- 
ture. Professor Leslie says in this connection that while the 
temperature itself advances uniformly in arithmetical progression 
the increased dissolving power thus communicated to the air ad- 
vances with the accelerating rapidity of a geometrical progression. 

While experiments that have been made to test this theory have . 
not agreed in confirming its truth, they suggest that it may be at 
least approximately true. 

Sir Boverton Redwood states with regard to 86° B. (sp. gr., 
.6481) gasoline that 

100 volumes of air at 32° F. will retain 10.7 per cent of vapor, 
(9.7 per cent of the mixture). 

7 


calculated on the formula sp. gr. equals 


lowed in English books, is 





Unfortunately, 





186 ILLUMINATING ENGINEERING 


100 volumes of air at 50° F. will retain 17.5 per cent of vapor, 
(14.9 per cent of the mixture). 

100 volumes of air at 68° F. will retain 27 per cent of vapor, 
(21.3 per cent of the mixture). 

In this connection Redwood goes on to say that “air charged 
with 735 grains of gasoline per cubic foot has been found to pos- 
sess an illuminating power of 16.5 candles when consumed at the 
rate of 314 cubic feet an hour in a 15-hole Argand burner.” 

If we assume the gasoline vapor to have a specific gravity of 3., 
it follows that the mixture has 3114 per cent of gasoline vapor 
by volume. | 

Redwood goes on further to describe a series of experiments, 
which he carried on with the assistance of Mr. Blunderstone, to 
determine “the manner in which crude petroleum and certain 
volatile petroleum-distillates evaporate when subjected to a current 
of dry air. ... In these experiments, dry air was caused to bubble 
slowly through the liquid in a series of graduated tubes maintained 
at a constant temperature. ... A set of determinations being made 
at temperatures of 40°, 60°, 80° and 100° F.” 

At 60° three determinations were made with gasoline of a sp. gr. 
of .639, 44.7 c. c. of the liquid being used. In the first, 0.9 liter of 
air was passed through the six tubes; in the second, 2.15 liters, and 
in the third, 3.55 liters. The first gave a total evaporation of .66 
volume of liquid to 100 volumes of air; the second gave .59 and the 
third .51 volume. 

It is thus seen that the relatively small amount of air took up the 
largest amount of gasoline. The result of the first test, if calculated, 
shows that the mixture contained 53 per cent by volume of the 


gasoline vapor—certainly an extraordinary result. The probabilities 


are, at least in this last series of experiments, that the small quan- 
tity of air slowly bubbling through the liquid in six small streams 
resulted in a selective evaporation. If so, this does not truly repre- 
sent the result from a liquid of .639 sp. gr. Certainly, we are not 
warranted in believing that any such percentages of gasoline can 
be carried in air-gas practice as are indicated in the two cases last 
quoted. ; 

The limits between which gasoline vapor and air form an ex- 
plosive mixture are 2 per cent of vapor with 98 per cent of air and 
5 per cent of vapor with 95 per cent of air by volume. This fact 
furnishes a reason for dividing carburetted-air gas into two classes: 


GAS AND OIL ILLUMINANTS 187 


First, that in which the proportion of gasoline vapor to air is less 
than 2 per cent; and, second, that in which the proportion of gaso- 
line vapor to air is more than 5 per cent. 

The former presents some very interesting features. A carburet- 
ted-air gas containing 114 per cent of gasoline vapor is low in 
heating value, is non-explosive, is non-asphyxiating, and yet, when 
used with a Welsbach mantle, furnishes a satisfactory light. It 
would appear that such a gas has much to recommend it. This 
class of air gas has been adopted in England to a considerable ex- 
tent for lighting country estates, audience halls, summer hotels, 
and the like. As yet it has received little recognition in this coun- 
try. A company is now presenting its claims for recognition. 

The specific gravity of the vapor of gasoline, as now generally 
used for air gas, is about 3. 

The calorific value of gasoline is variously quoted. In this con- 
nection it is to be remembered that “ gasoline” is not a substance 
of constant chemical composition. Furthermore, the statements do 
not always show whether the value quoted is gross or net heating 
value. The United States Geological Survey gives 19,200 B. t. u. 
per pound as the net value of gasoline of .71 to .73 specific gravity. 
Bulletin No. 191 of the United States Department of Agriculture, 
on the authority of Lucke & Woodward, gives 21,120 gross, 19,660 
net, B. t. u. per pound. 

Redwood, in discussing vapor tensions, says: 

“ Salleron & Urbain give also the following as the determined vapor- 
pressures (vapor-tensions) of petroleum products of various densities.” 

He then goes on to say that the values given are “ founded on 
a belief not in all cases correct.” 

This table, so rather guardedly quoted by Redwood, gives as the 
vapor tension of distillate of sp. gr. .65 (B. 85.38), 2110 mm. of 
water. This can be accepted at least as approximately correct, and 
would then show that air would be saturated when 20.42 per cent 
by volume of gasoline was present. 

In this country the use of carburetted air has been confined for 
many years to machines that produce a mixture containing over 
5 per cent of gasoline vapor, and it has been the practice to use 
514 to 61% gallons of gasoline to 1000 feet of the mixture, the con- 
tent of gasoline vapor then showing a wide margin of safety above 
the 5 per cent explosive limit. A 514-gallon gas burned in an 
Argand burner, gives from 15 to 16 candie-power, it contains about 


188 ILLUMINATING ENGINEERING 


1314 per cent gasoline vapor, and its specific gravity is about 1.26. 
The specific gravity of the mixture is important; for being heavier 
than air, in case of leak, not possessing the tendency to rise, it is 
less rapidly dissipated by the ordinary means of ventilation. This 
necessitates increased precautions against explosion and asphyxia- 
tion. Such a gas cannot be subjected to 4 temperature below 
43° F. without depositing gasoline in the pipes; therefore, it must 
be protected against cold either by wrapping the pipes or by ex- 
ternal heat. This gas will have a calorific value of about 570 
B. t. u. per foot, and therefore can be employed to advantage for 
lighting (especially by mantles) and heating. 

Four principal systems, the first substituting hydrogen for air, 
are used in the application of gasoline vapor to gas making, and 
these are as follows: 

1. Although not an air-gas system, it may be convenient to men- 
tion here, by reason of similarity of method, the process of forcing 
manufactured hydrogen gas over or through gasoline by which the 
hydrogen, which has no illuminating value of its own, becomes 
saturated with the rich hydrocarbon vapors. This mixture has a 
high heating value ; and especially when used with the incandescent 
mantle, a high illuminating value. This system is seldom found 
in general practice and is principally used in metallurgical labo- 
ratories. 

2. The employment of devices by means of which a current of 
air is forced over or through a body of gasoline or some porous 
or fibrous material saturated or impregnated with gasoline, by 
which means the air becomes carburetted with the hydrocarbon 
vapors to such an extent that the mixture can be used advan- 
tageously for illuminating and heating purposes. This method is 
called the cold-air process, and is the one most used in small private 
installations and town plants. 

Fig. 10 shows such an installation. It consists of a blower “ A,” 
carbureter “LL” and mixer “MM.” The blower, operated by sus- 
pended weights, as shown in the drawing, or water power, takes 
in air and forces part through the carbureter and part into the 
mixer. 

Fig. 11 shows a sectional view of a box-type carbureter, the kind 
generally used in plants of moderate size. It is a flat, rectangular 
box made of sheet metal, having partitions running longitudinally 
and parallel to each other through the box, but leaving a connect- 


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GAS AND OIL ILLUMINANTS 


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190 ILLUMINATING ENGINEERING 


ing opening between each two adjacent compartments sequentially 
at alternate ends. In these compartments are hung or stretched, 
as shown, strips of Canton flannel. There is an opening for filling, 
an inlet for air from the blower at one end, and at the other end an 
outlet for the carburetted air. The carbureter is about 15 inches 
deep but is filled with gasoline to a depth of only 6 inches. It is 
buried in the ground. The air entering through the top at one end 


o Vent 


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CREE: NBM SER UR Pe 

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re ear ane ee ener oo Mes 
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lif i Ais oo ee ee 
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Canton Flanre/ 
Fig. 11.—Carbureter, 50-Light Air Gas Machine. 


traverses all the passages, flowing through the flannel which, by 
capillarity, is kept wetted with gasoline; the carburetted air then 
passing on to the mixer. The box. must be of sufficient size to per- 
mit a very slow movement of the air and the recovery from the 
surrounding earth of the heat rendered latent by the evaporation 
of the gasoline. 

As stated before, commercially refined gasoline is a mixture of 
several hydrocarbons, though largely composed of hexane and 





7 


Ce arburretted 
Arr Outlet », 


Gas AND OIL ILLUMINANTS 191 


pentane. Under the conditions of slow evaporation here presented, 
there is a selective evaporation, the lower boiling fractions going 
off in large volumes first, gradually decreasing in volume as the 
gravity of the remaining gasoline increases, until finally a mini- 
mum permissible candle-power is reached, when the carbureter 
must be recharged. The cycle is then repeated. To minimize this 
fluctuation in candle-power and heating value, a tank containing 
a considerable supply of gasoline is sometimes connected to the 
carbureter with a ball and float valve, by which the height of the 
gasoline in the carbureter is replenished as fast as evaporated. It 
is claimed that this is an unnecessary refinement when a separate 
mixer is employed. 

When air is brought so intimately in contact with highly volatile 
gasoline the quantity of gasoline vapor that passes off with the air 
may be considerably in excess of that required to saturate the air 
at the final temperature. The gas from the carbureter is, there- 
fore, not then in condition to use; it is too rich and unstable as to 
condensibility. As has been shown, and perhaps explained, Red- 
wood is authority for the apparently contradictory statement that 
while air will require only 22 per cent of gasoline vapor to saturate 
at 60° F., yet when the air is bubbled slowly through a series of 
six tubes containing gasoline of the same gravity at the same 
temperature, the mixture of vapor and air passing off consisted 
of more than 50 per cent vapor. 

The carburetted air from the carbureter is passed into the 
mixer (Fig. 10), The mixer consists of a small holder rising and 
falling above the water in an enclosing metal cylinder. The holder 
has trips which open and close cocks at its lowest and highest 
points, thereby operating automatically by the flow of the gas. 
There is a test light and an adjusting cock for regulating the pro- 
portion of air to be mixed with the highly carburetted air from 
the carbureter, and a valve which is designed to control within 
certain limits the proportions of air from the blower and carburet- 
ted air from the carbureter. 

This is known generally as the cold-air process; under proper’ 
and reasonable supervision it affords a safe and practical means of 
illumination and heating. When installed so as to comply with the 
underwriters’ requirements it involves no increase in insurance 
rates. While designed and intended only for a mixture above the 
explosive limits it could be mechanically adapted to yield a mixture 
below the explosive limits. 


192 ILLUMINATING ENGINEERING 


3. To convert gasoline into a vapor by the application of ex- 
ternal heat and then by suitable mechanical means to mix the gas 
or vapor so formed with any desired proportion of air. This 
process has been applied in a number of types of air-gas machines. 
Generally, the heating device is in the form of a coil through 
which the gasoline passes and which is heated by a burner. 

Machines of this class are simpler as to number and complexity 
of parts, but the direct application of flames to a coil containing 
gasoline has not been considered safe by most insurance companies, 
and their use is therefore restricted. 

4. The fourth method consists of inducing a current of air 
into a small tube by a jet of steam and at the same time allowing 
sufficient gasoline or naphtha to enter to condense the steam. and 
combine with the air. The latent heat of the steam in this process 
is intended to compensate for the refrigerating action of the gaso- 
line or naphtha in passing to the state of vapor. 

With both the third and fourth methods petroleum naphtha of 
a considerably lower gravity may be used, say 72° to 68° B. (sp. 
gr., .6931 to .7071) ; while with the cold-air process gasoline not 
heavier than 82° B. (sp. gr., .6604) can be used without the neces- 
sity of pumping the residue from the carbureter oftener than 
once in 6 months. 

The fourth method, one of the earlier inventions of Hiram 
Maxim, is probably best for a large output of gas. Fig. 12 shows 
one of these machines with a sectional view of the steam injector 
for air and naphtha. Steam at about 60 pounds gauge pressure, 
controlled by a regulator, is supplied to chamber “ A,” from which 
it issues at high velocity through injector nozzle “lL” into tube 
“Gq,” drawing in air from “C” by the injector action. At the 
other end of tube “G” a secondary injector action takes place, 
naphtha entering by the adjustable valve “D.” The latent heat 
of the steam vaporizes the naphtha and by doing so the steam 
itself becomes condensed. The naphtha vapor and air unite and 
pass into the gas holder, while the condensed steam is trapped away. 
The operation of this machine is entirely automatic. When work- 
ing close to its capacity very little of the gas remains in the 
holder, but when the consumption of gas is reduced to a minimum 
the holder fills with gas, and by means of a system of trips and 
levers the process is interrupted by the closing of the steam noz- 
zle; when the holder descends the operation is reversed, the steam 


GAS AND OIL ILLUMINANTS 193 


nozzle is opened and the making of gas continues as before. By 
regulating the adjustable air and naphtha valves any desired mix- 
ture of vapor and air can be obtained, and in larger quantities 
than with any of the cold-air processes. 

The simplicity of carburetted-air processes is evident; no puri- 
fying of the delivered gas is required, and all the heat of the 
liquid fuel is directly transferred to the air and vapor mixture. 

The burners used for securing illumination through the agency 























of carburetted air are the ordinary flat-flame lava tip, and the 
various forms, both upright and inverted, of mantle burners. 

Where no mixer is installed and the gas is consumed directly 
from the carbureter the lava-tip burner has a small set-screw, 
by which the gas can be adjusted in its flow so as to prevent heavy 
and smoky flames. 

The specific gravity of the gas being much greater than that 
“of coal or water gas, it requires a larger opening in the check for 
the same quantity of gas to flow through, and in some cases larger 
openings for the air through the Bunsen are required. 

When a well-designed mixer is installed with the machine, as it 


194 ILLUMINATING ENGINEERING 


always should be, there is no inconvenient fluctuation in the candle- 
power of the light from the mantle burner. 

When burning a mixture containing less than 2 per cent gaso- 
line—below the range of explosibility—the Bunsen burner on the 
Welsbach burner is omitted entirely, as the gas contains sufficient 
air for a non-luminous flame. 

The extent to which carburetted-air gas is used for lghting 
cannot be determined accurately from available statistics. It occu- 
pies a field similar to acetylene—that of isolated plants and plants 
for the general supply of small towns and villages. From many 
of the plants, especially those operated by municipalities, no 
answers are received to applications for information; in many 
other cases the answers are vague and ambiguous. Brown’s Gas 
Directory shows that in the United States there are 124 town — 
plants. It is claimed that, including the smaller plants, there are 
twice this number. A fair estimate of the amount of gas made 
and distributed by the 124 town plants is not less than 166,000,000 
cubic feet a year. The gas is used for street lighting as well as 
for domestic consumption. In some cases the gas is distributed 
through a considerable mileage of mains. ‘The prices charged 
vary from $1.25 to $2.50 per 1000 feet. One of the largest com- 
panies reports a total annual sale of 35,000,000 cubic feet sold 
through 126 meters and 44 public lamps and distributed through 
814 miles of mains. 

All things considered, perhaps the field in which carburetted- 
air gas can demonstrate its greatest economic efficiency is in that 
of factories using various special heating devices of comparatively 
small individual capacity. The plant being installed primarily 
for this special heating, it can also be employed economically for 
lighting. 


Acetylene 


Acetylene is one of the group of hydrocarbons covered by the 
general formula C,,H,,, its own formula being C,H,; that is, 
its one molecule contains two atoms each of carbon and hydrogen. 
This gas has long been known to the chemists; and even as pro-" 
duced synthetically, by uniting the elements in the compound, the 
record goes back to 1836, though the reaction was not then fully 
understood. In 1862 Woehler announced the discovery of the 


GAS AND OIL ILLUMINANTS 195 


production of acetylene from calcium carbide made by heating to 
a very high temperature a mixture of charcoal with an alloy of 
zinc and calcium. Acetylene was known by chemists, and gas 
engineers also, as one of the heavy illuminants analytically pro- 
duced in small percentages during the destructive distillation of 
coal in the making of coal gas and in the generation of water gas, 
and its high value as an enricher was understood. 

Acetylene polymerizes at about 600° C. (1112° F.), that is, at 
elevated temperatures it is converted into other hydrocarbons hav- 
ing the same percentage composition, but containing more atoms 
of carbon and hydrogen in their molecules. Acetylene readily 
polymerizes to benzene, C,H,. This change is indicated by the 
equation 3C,H,=C,H,. Benzene, like acetylene, contains by 
weight almost exactly 92.3 per cent carbon and 7.7 per cent hydro- 
gen, but its molecule contains six atoms of each element instead 
of 2, as in the case of acetylene. It will be seen later that this 
instability of acetylene, together with its other characteristics, has 
a most important bearing upon its treatment and application and 
the precautions to be taken against accidents. 

In 1892 Thomas M. Willson, an electrical engineer, while ex- 
perimenting on the production of metallic calcium, employing 
therefor an electric furnace of high voltage in which was a mix- 
ture of lime and coal-tar, obtained a mass which he accidentally 
discovered contained calcium carbide, and which gave off acetylene 
when immersed in water. Willson was the first to demonstrate 
that acetylene could be obtained from calcium carbide in sufficient 
quantities and at a cost that would secure it a place in the in- 
dustrial arts. 

This discovery of Willson’s undoubtedly increased and intensified 
the interest in electro-chemical research and in synthetic chemistry, 
which two fields of research hold out much of promise for the bene- 
fit of mankind. It has also served to strengthen the theory or sur- 
mise that metallic carbides exist in the earth’s interior, and are 
the origin of petroleum and natural gas. Calcium carbide is com-. 
posed of one part of calcium and two parts of carbon, as shown 
by the formula CaC,. It is a hard, crystalline substance, dark gray 
in color, specific gravity about 2.22. One cubic foot of compact 
carbide therefore weighs about 138 pounds. 

The two highly refractory substances, lime and carbon, are 
forced to combine under the action of excessively high tempera- 


196 ILLUMINATING ENGINEERING 


tures, as most readily obtained in the electric furnace. The re- 
action is shown by the equation 


C20 SRO 3c | CaC, x 


CO 
(Quicklime) (Carbon) ~ (Calcium Carbide) (Carbon Monoxide) 


which shows that 56 pounds of lime combine with 36 pounds of 
carbon to form 64 pounds of calcium carbide and 28 pounds of 
carbon monoxide. Roughly, then, for the making of a long 
ton of the carbide, there is required a short ton (2000 pounds) of 
lime and 1275 pounds of carbon. 

In the manufacture of the carbide the purity of the raw material 
is of prime importance. Those forms of carboniferous material 
in which there is a low percentage of fixed carbon are to be avoided 
as the rapid evolution of gaseous products therefrom is likely to 
lead to explosions. 

The calcium carbonates, such as limestone, marble, etc., from 
which the lime or calcium oxide is prepared, must be low in con- 
tent of magnesia, alumina, silica, sulphur and phosphorus. The 
ordinary limekiln cannot be used because of the impurities that 
‘would be introduced therefrom. As it takes about 100 pounds of 
carbonate of lime to yield 56 pounds of the oxide, those impurities 
not driven off with the carbonic acid would be nearly doubled. 
These necessary precautions led to the general practice of cal- 
cining the carbonate at the carbide factory. 

After mixing the lime and carbon in proper proportions they are 
fused by a powerful electric current. Resistance and are furnaces 
are both used. ‘The furnace must be operated under uniform 
heating. For the generation of the heavy currents required re- 
course may now be had to more or less remote water powers if other- 
wise desirable, as railroad transportation of the carbide is no 
longer hampered by onerous restrictions. The carbide is neces- 
sarily packed in tightly sealed cans to protect from moisture. 

While a generation has not yet elapsed since the first introduc- 
tion of acetylene to the commercial world the files of the patent 
offices contain such a multiplicity of applications, granted and 
rejected, that it would be futile at this time to touch on this branch 
of the subject. Many of these applications show that the inventors 
neither understood the principles involved nor the progress of 
the art, an ignorance frequently accompanying much so-called 
invention. 


Oe 


GAS AND O1L ILLUMINANTS 197 


The production of carbide in Europe in 1908 is approximated 
as follows: 





Tons 

mveden and Norway <=. cies mye. so %u. 35,000 
PELAUNOG cece ch ansls ERNE waiters er cate Fore 26,000 
PoP ETC na eee Pet Erg oo he sk 30,000 
UL Uae ho Sh aes et ae a ot Ea SS ERS Ce 31,000 
RMA Mette ae cree nee aha! get cha wh Uae RE 20,000 
Rata R Cote g Wa aos os alc laces u's ofiphess Tite hence 40,000 
PP OLCOICT dee sac Bi duck eke ks ete s Ue 10,000 
MarR tress crete, «eek Ra exe's © arena Te 192,000 


Practically all of this carbide was used for the production of 
acetylene. 

Coming now to the manufacture of acetylene, it is to be regretted 
that more complete and accurate data cannot be had as to its use 
as an illuminant, and especially in the United States. Brown’s 
Directory of Gas Companies records 184 acetylene town plants in 
operation the first of this year. hese works report a total output 
of 18,500,000 cubic feet. A paper read before the Iiluminating 
Engineering Society in 1909 is authority for the statement that 
there were at that time 290 towns lighted with acetylene. It can — 
be understood readily that the record in Brown’s Directory, de- 
pending for its facts as it does upon answers to question sheets, 
may be quite incomplete by reason of the indifference of those 
in control, and especially so in case of the municipal plants. 

In addition to the acetylene so distributed, the total is con- 
siderably increased by that used in private houses, contractors’ 
plants, car lighting and portable lamps, particularly automobile 
search-lights. 

The rate charged for acetylene by the town companies seems to 
run from 11% to 2 cents per cubic foot, or $15 to $20 per 1000 
eubie feet. Under efficient management, as to installation and 
operation, these rates are said to afford a fair return on the in- 
vestment. 

To comprehend the precautions to be taken in the use of calcium. 
carbide and acetylene, there must be borne in mind the difference 
between exothermic and endothermic reactions. 

Exothermic compounds are those whose formation from ele- 
mentary substances is attended with liberation of heat, and whose 
decomposition into simpler compounds or elementary substances 
is attended with absorption of heat. 


198 ILLUMINATING ENGINEERING 


Endothermic compounds are those whose formation from ele- 
mentary substances is attended with absorption of heat, and whose 
decomposition into other compounds or get te substances is 
attended with liberation of heat. 

These latter compounds are not very numerous, they are more 
or less unstable, and some of them are resolved into their elements 
with explosive force. 

Acetylene 1s an endothermic compound. 

Acetylene is obtained from calcium carbide through a double 
decomposition. “The first step is shown by the equation 


CaC, a HO 
(Calcium carbide) (Water) 


Si G,Ho) CaO 3 
™ (Acetylene) (Calcium oxide or lime): (1) 


But the quicklime, CaO, in the presence of an excess of water, 
will be found in the form of slaked lime, or calcium hydroxide, 
Ca(OH)., as shown by the equation 


CaO0+H,O=Ca(OH),. (2) 


As these reactions in the presence of sufficient water may occur 
simultaneously, the double reaction can be shown by the equation 


CaC, +2H,0—C,H, + Ca(OH),. (3) 


This is an exothermic reaction because the quantity of heat lb- 
erated exceeds the quantity of heat absorbed. There is some little 
question as to the heat of formation of calcium carbide, authori- 
ties varying from —0.65 calories (large) to +3.9. But these 
differences of opinion do not affect the question as to whether the 
reaction as a whole results in absorption or liberation of heat; 
it only affects in minor degree the quantity of heat liberated. 

The heat of formation of Ca(OH), (exothermic substance) is 
+160.1 large calories; the heat of formation of water (exothermic 
substance) is +69, and hence for decomposition is —69; taking 
heat of formation of calcium carbide as +3.9, for decomposition 
it is —3.9. The heat of formation of acetylene is —58.1. As the 
formation of Ca(OH), is obtained by the decomposition of the 
water and the carbide and the formation of the acetylene, we have 
heat liberated in the formation of the Ca(OH), 160.1, and the 
heat absorbed as follows: 


GAS AND O1L ILLUMINANTS 199 


Formation of acetylene — 58.1 Endothermic substance. 
Decomposition of water — 69. Hxothermie. 
Decomposition of carbide — 3.9 Exothermic. 

Total —131.0. 


Deducting the 131 absorbed, from the 160.1 set free, we have as 
a net result 29.1 large calories liberated. 

While this reaction as a whole is exothermic, acetylene as a 
substance is seen to be decidedly endothermic, and so is ready to 
liberate large quantities of heat whenever the conditions for de- 
composition obtain. 

While this reaction may be modified it should be pointed out 
that the reaction where there is no excess of water, as indicated 
in equation (1), produces in practice results which are quite dif- 
ferent from those obtained where there is excess of water, as indi- 
cated in equation (3). 

In the acetylene generators of the most modern and usual pat- 
tern, some of the surplus water is evaporated by the heat liberated, 
and some of this water vapor, even at low temperatures, is carried 
away with the escaping gas. If the heat liberated during the de- 
composition of the carbide is not otherwise absorbed, it is sufficient 
in amount to vaporize almost exactly three parts by weight of 
water for every four parts of carbide attacked. But if this quan- 
tity of heat were expended upon some substance, such as acetylene 
or calcium carbide, which, unlike water, cannot absorb an extra 
amount by changing its physical state, as from liquid to gas, the 
heat thus generated during the decomposition of the carbide would 
be in evidence to a far greater extent. For reasons that can be 
indicated only within the time allowed me, it is essential for good 
working that the temperature of both the acetylene and the carbide 
shall be prevented from rising to any considerable extent. 

Experiments were conducted by Caro and by Lewes to determine 
the temperature of the carbide due to decomposition. Caro’s ex- 
periments showed a maximum temperature of 280° C. (536° F.). 
Lewes’ experiments gave a maximum temperature of 807° C.. 
(1480° F.). The temperature attained is in part dependent upon 
the time elapsed in the reaction, for the longer the time the greater 
the opportunity for the escape of heat liberated. The divergence 
in the results obtained by Caro and Lewes is explained by the 
difference in the design of the generators and the speed at which 
they were operated. In Lewes’ generator little or no provision was 


200 ILLUMINATING ENGINEERING 


made against overheating, and it is not to be supposed that such 
temperatures as were observed by Lewes are found in a commer- 
cial generator. But his determination is important as showing the 
danger to be avoided, for the temperature he found is considerably 
above that at which acetylene decomposes into its elements in the 
absence of air, namely, 780° C. or 1436° F. Excessively high tem- 
peratures in the generator must be avoided, because whenever the 
temperature in the immediate neighborhood of a mass of calcium 
carbide which is evolving acetylene under the attack of water rises 
materially above the boiling point of water, one or more of three 
objectionable effects is produced; namely, upon the gas generated, 
upon the carbide decomposed,.or upon the general chemical re- 
action then taking place. Time does not permit a full discussion 
of the questions here involved, but a few hints may be given. 

Lewes points out that not only does acetylene decompose at 
780° C., but it begins to polymerize at 600° C. (1112° F.). Sup- 
pose acetylene polymerizes into benzene, the burner adapted to the 
efficient utilization of the former will not be so adapted for ben- 
zene. Furthermore, under certain conditions, the benzene liquefies 
and deposits with water vapor in the pipes. An additional trouble 
from polymerization occurs when the temperature rises above the 
point at which benzene is formed, for then other hydrocarbons may 
be formed having a higher proportion of carbon than is present 
in acetylene and benzene, setting free non-luminous hydrogen, and 
thus reducing the illuminating value of the gaseous mixture. In 
certain experiments by Lewes the loss in candle-power was found 
to be a reduction from 240 to 126. Another effect of heat upon 
acetylene has already been indicated. Being an endothermic sub- 
stance it gives out heat upon decomposing. It decomposes at 
780° C. when free from air, a spark, or shock, or pressure of 30 
pounds or more being sufficient to effect the change. This change 
raises the temperature and so increases the pressure of the disasso- 
ciated hydrogen, and may cause the containing vessel to explode. If 
air is present, as it may be through bad design of apparatus or 
incompetent attendance, the acetylene can be ignited at 480° C. 
(896° F.). Under certain conditions 25 per cent of air and 75 
per cent of acetylene are explosive. 

The extreme limits of explosibility of acetylene mixed with air 
are variously stated. Clowes gives the extremely wide range of 
explosibility from 3 per cent to 82 per cent of acetylene. Le 


GAS AND OIL ILLUMINANTS 2()] 


Chatelier gives 2.9 per cent to 64 per cent. Hitner made exhaustive 
tests with several gases, in each case the mixture being saturated 
with aqueous vapor, thus reducing the limits of explosibility. For 
acetylene he gives from 3.35 to 52.30 per cent. Teclu, experi- 
menting with a dry mixture, determined the limits as 1.53 to 59 
per cent. These results naturally are changed if the mixture con- 
tains other gases besides acetylene and air, but enough has been 
said to show that acetylene cannot be handled carelessly. This 
is emphasized by Hitner’s experiments, comparable but not giving 
extreme limits, which gave as the limits for coal gas 7.90 to 19.10 
per cent, or a range of only 11.20 per cent against acetylene range 
of 48.95 per cent, as shown above. 

In the generator the effect of heat on the carbide itself may be 

troublesome. If part of the gas polymerizes part may so be re- 
solved into tar, which coats the carbide still unattacked and so 
protects it more or less from further attack, thus reducing the 
output and leaving the residue with a content of acetylene, which 
may later occasion trouble during or after removal. 
- The effect of accumulating heat in the generator itself has to 
be guarded against. For example, at a temperature as low as 
200° C. (392° F.), if the ordinary solder were used in the joints 
it would be melted and the vessel become unsafe. This serves to 
point to the fact that the materials used and the minor details of 
construction in a generator may be such as to condemn a design 
generally commendable. 

Having indicated most superficially some of the conditions to 
be considered in the design and construction of acetylene genera- 
tors, with the aid of diagrams taken from Leeds and Butterfield’s 
work entitled “ Acetylene, Its Generation and Use,” I shall show 
in a general way how these conditions are met, but without at- 
tempting to discuss the relative advantages and disadvantages of 
the several types. 

Acetylene generators may be roughly classified as follows: 


1st. Carbide to water. 
(a) Non-automatic. 
(b) Automatic. 


2d. Water to carbide. 
(a) Non-automatic. 
(b) Automatic. 


202 ILLUMINATING ENGINEERING 


In general, the type having the widest limits of safety is that in 
which a small quantity of carbide is introduced into a considerable 
body of water, the acetylene as it bubbles through the water passing 
directly out and into a holder. If this holder has ample capacity 
for the maximum night’s demand, it can be filled with gas during 
the day and the generator locked for the night. This non-auto- 
matic form may be criticized on the ground of first cost. 


Ce2he 






Cale 


wi Wie. 18: Fie. 14, 


Fic. 13.—Acetylene Generator. Non-Automatic. Carbide to Water Type. 
Fic. 14.—Acetylene Generator. Automatic. Carbide to Water Type. 


If the introduction of carbide is controlled by an automatic 
device which admits carbide automatically as the acetylene is con- 
sumed, a smaller generator and holder can be employed. 

Figs. 13, 14 and 15 show types of carbide to water generators. 

Fig. 13 represents the non-automatic type. The carbide is fed 
by hand through the chute A into the generator B. The generator 
is filled with water above the opening of the chute to prevent the 
gas from escaping through the chute. Grids D and E catch and 
support the lumps of carbide, permitting the acetylene to be com- 


GAS AND OIL ILLUMINANTS 203 


pletely liberated before permitting the mass to mix with the sludge 
of slaked lime in the bottom of the tank. The carbide cannot be 
used in small lumps, as then the generation of acetylene would be 
sufficiently active to blow the seal and allow the gas to escape 
through the chute. 

Fig. 14 shows an automatic generator of the first class. The 
carbide is held in a hopper which is supported by holder bell I, 
which rises and falls according to the volume of acetylene con- 
tained. The hopper is closed at the bottom by a valve G, from 











~ 
‘ 





Fic. 15. Fig. 16. 


Fia. 15.—Acetylene Generator. Automatic Dipping. Carbide to Water 
Type. 

Fic. 16.—Acetylene Generator. Water to Carbide Type. Water Inlet at 
Top. 


which depends a rod H. As acetylene is withdrawn from the bell 
the bell falls until the rod strikes the bottom of the tank, the valve 
is thus forced open permitting more carbide to fall into the water, 
more acetylene is released, the bell again rises until the valve seat 
and valve engage, when the supply of carbide is again stopped. 
Fig. 15 shows a dipping generator. ‘The carbide is held in a 
perforated vessel which hangs from the inside of the crown of the 


204 ILLUMINATING ENGINEERING 


holder bell. As the acetylene is consumed the bell falls until the 
carbide dips in the water, when acetylene is again liberated. 

Figs. 16, 17 and 18 show types of water to carbide generators. 
Fig. 16 shows a generator in which the carbide is contained in a 
series of pans, P, P1, P2 and P3, a small quantity in each pan. 
Water is admitted at the bottom through pipe M. As each pan 
is flooded the acetylene rises to the top of the tank and passes out 


fi2 0 C2 He 





A SS en ean ee 


Fiq. 17.—Acetylene Generator. Water to Carbide Type. Water Inlet 
at Top. 


at R. The gas passing out is charged with water vapor, and this 
water acting upon the carbide in the upper pans produces “ after 
generation,” which is an objectionable feature. 

Fig. 17 shows a better type. The carbide is contained in pans 
as in the previous case. Here the water is admitted at the top 
of the tank and first acts on the carbide in the top pan. The gas 
passes off without coming in contact with the carbide in the other 
pans. As the first pan is flooded the water overflows through the 
pipe S to the second pan. This is repeated until the carbide in the 
last pan is attacked. The aeetylene escapes from the pipe at the 
top of the tank, as shown. 


i ie - 


Gas AND OIL ILLUMINANTS 205 


Fig. 18 shows a generator not to be commended. The carbide 
is contained in the tank T. Water enters at the top in drops or 
a fine stream. This type produces “after generation” and dan- 
gerous overheating. 

Generally speaking, in the water to carbide generators the gen- 
erator is opened to the air while being charged with fresh carbide; 
this is a decided disadvantage, for, as already shown, acetylene 
should be guarded from mixing with air on account of its wide 
range of explosibility. 





Fig. 18.—Acetylene Generator. Water to Carbide Type. Crude Form. 


What I have said fails to show the great variety of apparatus 
actually employed or the manner in which the several types merge 
into each other. I have not attempted to show the complete acety- 
lene installation, including the parts for generation and governing. 
It should be pointed out that it is necessary either to use a pure 
carbide or provide means for purifying the acetylene, as otherwise 
compounds of phosphorus, silicon, ammonia and sulphur might 
be present rendering the gas objectionable on the score of spon- 
taneous inflammability or non-hygienic qualities. Leeds and But- 
terfield’s work give the rules and regulations adopted by govern- 
ments and insurance companies for the construction and installa- 
tion of acetylene plants. 

The carbide is sold in several sizes. For generators the size 
varies from 314 inches by 2 inches down to 14 inch by 1/12 inch. 
For lamps, from 1 inch by 1% inch down to dust. The rate of 


206 ILLUMINATING ENGINEERING 


evolution is inversely proportional to the size of the lump. Lumps 
coated with dust may give irregularity in operation. 

Acetylene liquefies at 0° C. and about 211% atmospheres pres- 
sure. It is then most unstable, spontaneous disassociation with 
explosive force being due to occur on the application of a spark or 
when shocked. After quite a number of disastrous accidents it is 
now generally understood that liquid acetylene is too dangerous 
to use. As before mentioned, the gas is liable to explode if heated 
to 780° C. or if held under a pressure of 2 atmospheres absolute, 
or above. 

Acetylene is readily soluble in many liquids, and this property 
is utilized to bring the acetylene into small compass. Acetone, at 
ordinary temperature and atmospheric pressure, will dissolve about 
25 volumes of acetylene, and at 12 atmospheres will dissolve about 
300 volumes. Acetone is an exothermic substance with a composi- 
tion shown by the formula C,H,O, and hence combustible, and 
within certain limits of pressure its presence tends to decrease the 
severity of explosion. At 20 atmospheres pressure the acetone adds 
to the danger from explosion. Acetylene dissolved in acetone car- 
ried up to a pressure of 10 atmospheres is safely employed, but 
there are practical objections to its use in this liquid form. ‘To 
overcome these objections the cylinders are filled with some porous 
material which does not react on the acetone. A material is used 
which has a porosity of 80 per cent, that is, when the vessel is 
apparently full of the material about 20 per cent only of the space 
is really occupied. 'The portable cylinders for this service cannot 
be filled with acetone, for the reason that ample space must be left 
for expansion as the liquid takes up the gas. A cylinder having a 
normal capacity of 100 volumes will have say 20 volumes taken 
up by the porous filling, and can safely be charged with 40 volumes 
of acetone. This 40 volumes of acetone dissolves 40x 25=1000 
volumes of acetylene; and by compression to 10 atmospheres this 
is increased to 10,000 volumes. In this form acetylene is sold 
under various trade names and used for automobile head lights 
and similar service where limited storage capacity is of decided 
moment. ; 

Acetylene, under favoring conditions including moisture, will 
combine with copper to form acetylide of copper, an explosive 
compound. As acetylene is now generally produced and used these 
conditions are not apt to obtain, so the danger from this source is 


= Pee ee ee, ee ee —- 


GAs AND O1L ILLUMINANTS — 20% 


now not regarded seriously. Copper alloys and compounds should 
not be employed in the construction of parts of plant exposed to 
the gas or in the process. 

Straight acetylene, burned in an open-flame burner of a char- 
acter and size best adapted to give the highest illuminating value, 
the burner being so placed as to carry to the photometer disc the 
strongest horizontal rays, the bar readings being calculated pro rata 
to a consumption of 5 feet an hour, gives a candle-power of from 
240 to 250. 

The general practice of selecting the burner and rate of con- 
sumption so as to develop best efficiency of the gas instead of being 
confined to one type of burner and a rated consumption of 5 feet 
an hour, has received the approval of the Gas Referees of London 
acting under Parliamentary powers. 

The specific gravity of acetylene is .9056, usually taken as .91. 

For self-luminous flames, lava-tip burners are employed, the 
gas issuing either from a slot or two holes, both producing flat 
flames. With the latter form the flat flame is produced by the 
impinging of the two currents of gas against each other, the plane 
of the flame being produced at right angle to the plane of the two 
holes. The burners are made in many different forms in the effort 
to overcome difficulties due to the richness of the gas and its 
instability. The richness of the gas made it necessary to employ 
small burners or to make extra provision for injecting air into the 
body of the flame by the action of the issuing gas. This was best 
accomplished by some form of two-jet burner, which dragged in 
the air at a point between the jets and below the flame. To better 
accomplish this result burners were devised with two tips so as to 
separate farther the two jets of gas. Further, to assist in the in- 
jection of air, acetylene is burned at a pressure far in excess of 
that employed with coal gas. Its high specific gravity also calls 
for additional pressure. The design of acetylene burners well il- 
lustrates that burners must be designed to supply such a quantity 
of air to the flame as will produce a maximum incandescence. If - 
one of these burners were used with coal gas, so much air would 
be dragged in that the carbon particles of this thinner gas would 
be consumed with little or no preliminary incandescence. 

The comparatively high efficiency of the acetylene flame is due 
not alone to the high carbon content; an important factor is the 
high flame temperature, which is in part the result of liberation 


208 | ILLUMINATING ENGINEERING | 


of heat at time of disassociation of this endothermic gas. Mahler 
gives the flame temperature at 0° C. and 760 mm. as 2350° C. or 
4642° F. Le Chatelier gives 2100° C. to 2400° C. 

Acetylene is also employed with incandescent mantles, resulting 
in a considerable increase in candle-power, this gain according to 
different authorities being from 160 to 200 per cent. For certain 
special applications a still larger gain has been secured. ‘There 
are decided difficulties to be overcome and advantages to be aban- 
doned in using acetylene for incandescent lighting, and the high 
efficiency and the whiteness of the self-luminous flame make it 
less necessary or desirable to overcome these difficulties. 

Acetylene is also employed for illumination in the form of car- 
buretted acetylene or carburylene, and in this form it is more ad- 
vantageously apphed to incandescent lighting, but time does not 
permit a discussion of this branch of the subject. 

In connection with illuminating engineering, the color of the 
acetylene flame is of great importance. A comparison with sun- 
light and other light sources will be given in another of these 
lectures. 7 

Acetylene can also be used for heating. Its calorific value per 
foot is 363 large calories, or 1440 B.t.u., which is about two and 
one-half times that of city gas. The comparison is not favorable 
to acetylene, however, when relative costs are considered. 

Within the limits of a single lecture, inordinately long, it is 
true, I have, according to instructions, endeavored to cover three 
sources of illumination. Many lectures could be devoted advan- 
tageously to each of these. Acetylene alone could not be covered 
completely in many lectures. 


BIBLIOGRAPHY 


PINTScH GAS 


King’s Treatise on the Manufacture of Gas. Volume III. 

The Comparative Merits of Various Systems of Car Lighting: A. M. 
Wellington, W. B. D. Penniman, Charles Whiting Baker. Engineer: 
ing News Publishing Company, New York, 1892. 

Engineering Chemistry: Thomas B. Stillman. The Chemical Publish- 
ing Co., Easton, Pa., 1910. 

Car Lighting: R. M. Dixon. Stevens Institute Indicator, Vol. XXV, 
No, 1). Jan., 1908: 

Lighting of Railway Cars: Geo. E. Hulse. Transactions of the Illumi- 
nating Engineering Soc., Vol. V, No. 1, January, 1910. 





GAS AND OIL ILLUMINANTS 209 


Car Lighting: L. R. Pomeroy. Proceedings Canadian Railway Club, 
Vol. IX, No. 2, February, 1910. 

Leuchtfeuer und Nebelsignal: E. Klebert. Journal fur Gasbeleuchtung 
und Wasserversorgung, May, 1909. 

Oelgasaustalt mit Generatorbetrieb: Fritz Landsberg. Zeitschrift des 
Vereines Deutscher Ingenieure, Nr. 37, Band 52, September, 1909. 

Oelgasherstellung in Generatoren und Gasfermversorgung in Hoch- 
druckleitung: Fritz Landsberg. Glaser’s Annual, August 1, 1910. 

Lighting of Passenger Cars: Max Buettner. Published by Springer, 
Berlin, 1901. 

Petroleum and its Products: ‘Vol. II, Sir Boverton Redwood. Published. 
by Charles Griffin & Co., Ltd., London, England, 1906. Brief descrip- 
tion under oil gas. 


AIR GAS 


Petroleum and its Products: 2 Vols. Sir Boverton Redwood. Pub- 
lished by Charles Griffin & Co., Ltd., London, England, 1906. This 
work contains a very full bibliography. 

Petrol Air-Gas: Henry O’Connor. Published by Crosby Lockwood & 
Son, London, England, 1909. 


ACETYLENE 


Acetylene: The Principles of Its Generation and Use by F. H. Leeds 
and W. J. Atkinson Butterfield. Published by Charles Griffin & Co., 
Ltd., London, England, 1910. 

Calcium Carbide and Acetylene by Geo. Gilbert Pond. Bulletin of the 
Department of Chemistry of the Pennsylvania State College, 1909. 
This work contains a full bibliography. 


on 


as 


vor 











AUER VON WELSBACH 





V (2) 
INCANDESCENT GAS MANTLES 
By M. C. WHITAKER 


CONTENTS 


INTRODUCTION 
~ Heat sources: chemical, electrical. 
Combustion. 
Substance: gas, wood, coal, etc. 
Supporter of combustion: oxygen, air. | 
Kindling temperature: electric spark, lighted match, etc. 
Chemistry of combustion. 
Marsh gas + oxygen — water vapor + carbon. 
Carbon + oxygen = carbon dioxide. 
Open tip combustion. 
Bunsen burner combustion. 


INCANDESCENT GAS ILLUMINATION 


Principles involved. 
History: Hare, Drummond and Claymond lights, Siemens-Lungren 
lamp, Auer von Welsbach lamp. 
Bunsen burner: history, construction and chemistry of operation. 
Adaptations for use with incandescent mantles. 
Upright and inverted. 
Single, cluster and arc. 
Inside and outside. 
Upright burner construction. 
Bunsen tube. 
Check for gas; plate, needle, multiple hole, check; air adjustment; 
gauzes; gallery. 
Inverted burner construction. 
Types: vertical, horizontal and goose-neck burners; velocity, 
gravity and buoyant action in downward flow of mixture. 
Checks for gas; air adjustment; means for overcoming flash-backs; 
crown for glassware. 


GAS MANTLES 


Process of manufacture: history, knitting, washing, saturating, incin- 
erating, shaping, collodionizing, trimming and inspecting, packing 
and shipping. 

Physical structures of mantles. 

Basic fibers: cotton, ramie, artificial silk. 
Threads, weaves, stitches, etc. 


a1? ILLUMINATING ENGINEERING 


Chemicals and sources. 
Lighting principles; thorium and cerium. 
Thorium; source (monazite, thorianite); manufacture, market, use. 
Collodion; composition, manufacture, use. 
Types of mantles: upright and inverted, railroad train, sizes, pressure, 
rag, acetylene, kerosene, etc. 
Quality and service characteristics as determined by process of manu- 
facture. 
Cotton: shrinkage, depreciation in candle-power; color value; 
physical strength. ' 
Ramie: ditto, etc. 
Artificial silk: ditto, etc. 


Introduction 


Assuming that the illuminating power of a gas flame is derived 


from the heating of solid particles to incandescence, the practice of _ 


gas illumination divides itself into two general principles: 

First. Where the solid incandescent material is supplied by the 
decomposition of the gas in the process of combustion. (Open-tip 
flame. ) 

Second. Where the gas is completely consumed in a Bunsen 
burner for the production of the maximum amount of heat and 
a permanent incandescent material is supplied as a part of the 
apparatus. (Incandescent gas system.) 

The first steps toward the improvement of the efficiency of gas 
for lighting was made on the first of these principles by pre- 
heating the gas before it reached the point of combustion in the 
so-called regenerative burner of the Siemens-Lungren or Gordon- 
Mitchell type (Fig. 1). There are some of these regenerative 
lamps in use at the present time. The regenerative burner was 


the most effective ever produced by following the first principle - 


mentioned above, and gave the most efficient results up to the in- 
troduction of the incandescent mantle system, which is based on 
the second principle. 

Professor Robert Hare (Philadelphia Chemical Society, 1802) 
first fully described a form of “ incandescent” gas light, which is 
the basic principle now utilized in this industry. 

At a meeting of the Philadelphia Chemical Society, held in 
December, 1801, he showed experiments and described this in- 
candescent lime light as follows: 

“The cock of the pipe communicating with the hydrogen gas was 


then turned until as much was emitted from the orifice of the cylinder 
as when lighted formed a flame smaller in size than that of a candle. 


Le ne en, a 


INCANDESCENT Gas MANTLES 213 


Under this flame was placed the body to be acted on, supported either 
by charcoal, or by some more solid, and incombustible substance. The 
cock retaining the oxygen gas was then turned until the light and heat 
appeared to have attained the greatest intensity. When this took place, 
the eyes could scarcely sustain the one, nor could the most refractory 
substances resist the other.” 





Fig. 1.—Regenerative Lamp. Fie. 2—Drummond Calcium Light. 


Henry Drummond, in 1826, made use of the incandescent lime 
light, similar to that suggested by Professor Hare, for signaling. 
Drummond’s application of this principle of producing an illumi- 
nation of high intensity was adopted generally, and he is usually 
eredited with the invention. The lime light is sometimes called 
the “ Drummond light” (Fig. 2). ere, 

At the Crystal Palace Exposition in Paris in 1883 a lamp of the 
inverted type was shown in which the illumination was produced 
by a platinum basket suspended in a blast flame. The life of the 
basket was limited to 50 or 60 hours. 

Various other lamps for the application of the principle of sup- 
plying the incandescing material were suggested, such as cones 


214 ILLUMINATING ENGINEERING 


made from platinum wires covered with a refractory coating, per- 
forated baskets, grids placed above the flame of the fish-tail 
burner, ete. 

The greatest step in the development of a commercial incan- 
descent gas light was made by Dr. Carl Auer von Welsbach. In 
1886, Dr. Auer, while a student in the laboratory. of Professor 
Robert Bunsen, in Heidelberg, discovered that the ash formed by 
saturating a cotton fabric in a solution of erbium salts and burning 
out the organic matter would take the shape of the original fibers, 
and would adhere to form a mesh of considerable strength. This 
finely divided ash fabric, when suspended in the flame of a Bunsen 
burner, became intensely luminous. Erbium, however, produces 
green light. Nevertheless, the principle of forming an attenuated 
but closely adhering ash was established by Dr. Auer in these © 
experiments, and he immediately proceeded to develop this idea 
with a view to producing a commercially desirable light by heating 
oxide webs which he called “ stockings” or mantles. 

His early mantles were made from a mixture of lanthanum and 
zirconium oxides. The light given by this mixture was not sat- 
isfactory, and the investigation was continued until he discovered 
the wonderful luminescence obtained with a mantle made from 
the rare oxides of thorium and cerium. 


Incandescent Burners 


The earlier burners constructed to use Auer’s invention were de- 
signed for use with the lanthanum-zirconium mantle, which did 
not give the high candle-power given by the present mantle. These 
burners were consequently very large and clumsy and hore a very 
remote resemblance to the modern types. 


Modern Types 


The present practice in incandescent burner construction should 
be divided, for clear discussion, into— 

Furst. Individual upright burners. 

Second. Individual inverted burners. 

Third. Gas arc lamps (upright burners). 

Fourth. Gas are lamps (inverted burners). 

Fifth. amps for special application (pressure oil lamps, rail- 
way coach lamps, kerosene lamps, etc.). 


INCANDESCENT Gas MANTLES 215 


Upright Burners 


The functional parts of the upright incandescent burner may be 
divided into (Fig. 3): 

(a) Bunsen tube. 

(b) Bunsen base. 

(c) Gas-adjustment means. 

(d) Air-adjustment means. 

(e) Mixing chamber. 

(f) Gallery to support chimneys, glassware, reflectors, etc. 


Mixing chamber. 


Gallery. 


Bunsen tube. 


Gas adjustment. 


Air adjustment. 


Bunsen base. 





Fig. 38.—Upright Burner Cut to Show Interior Construction. 


The Bunsen tube is carefully designed to meet a wide range of 
gas conditions, such as fluctuations in pressure, gravity, etc., and 
still produce a mixture which has entrained the proper quantity. 
of air to produce complete combustion at the gauze line. The 
dimeusions of this tube have’ been carefully determined in ex- 
perience, and are comparatively uniform in all styles of standard 
burners. 

The Bunsen base is usually turned from solid brass bar and 
threaded internally to fit the average run of 1£-inch gas nipples 


216 ILLUMINATING ENGINEERING 


rather than any standard thread. This base is also adapted to 
carry the gas-adjusting device and to form an assembly base for 
the entire lamp. 

A gas adjustment is an essential feature of the standard burner 
used in this country. Some foreign burners, and many of the early 
burners in this country, had a fixed gas orifice, but the variation 
in density and pressure of the gas in different localities have com- 
pelled the modern gas burner to include some means for acreage 
the gas flow. 

There are several prevailing types of gas checks, some of which 
fulfil the function of regulating the flow of gas, but fall far short 
of meeting other essential requirements. 

The efficiency of burners of this type is largely dependent upon 
the velocity of the gas jet, and its consequent ability to entrain the 
amount of air necessary to produce complete combustion. Any 
construction which tends to cut down this jet velocity seriously 
affects the efficiency and proper operation of the burner unless the 
initial gas pressure is high enough to produce a proper jet velocity 
in spite of the design of the check. Low and variable pressures are 
common conditions and, as a consequence, must be provided for 
in all designs intended for general sale and use. 

A single round hole through a thin plate offers the minimum 
amount of resistance to the flow of the gas stream and, as a con- 
sequence, gives the maximum jet velocity in the Bunsen tube. An 
iris diaphragm, similar to the device used in a camera, has been 
suggested as the ideal way to construct an adjustable single-hole 
check, but the cost of construction and the mechanical difficulties 
involved in making it gas-tight prevent its general adoption. 

Among the adjustable checks in general use the preferred types 
seem to be included in the following general designs: 

First. The Mason check, which is a series of round holes in 
superimposed plates, one of which may be rotated upon the other 
in such a way as to bring more or fewer holes into action, depending 
upon the direction of rotation. While the number of small holes 
offers somewhat more friction to the flow of the gas than the ideal 
single hole, it is thought that this device, which is capable of 
economical and reliable mechanical construction gives the most 
efficient results over the widest range of conditions. 

Second. The annular-orifice check is produced by inserting a 
needle in a single round hole and providing a mechanical construc- 


INCANDESCENT GAS MANTLES aa 


tion which permits the needle to be drawn in or out in relation to 
a stationary hole, or a stationary needle with a cap-orifice arranged 
to be raised and lowered. Obviously, the annular orifice, which 
may give satisfactory results with favorable conditions, will offer 
unfavorable resistance to the flow of the gas on lower pressures and 
thereby affect the mixture. 





Fig. 4.—Upright Burner. 


Adjustment of the air supply is usually automatically taken care 
of by the regulation of the gas flow when the composition and 
other conditions are normal. Certain gases require some extra 
provision for air adjustment, and most upright burners are now 
so constructed as to permit of this equipment, if necessary. 

The mixing chamber is the enlarged portion at the top of the 
Bunsen tube, and exercises an important function in producing . 
a more intimate mixture of gas and air, and also serves as a 
mounting for the mantle. 

The function of the gallery is obviously for supporting the 
chimney, globe, reflector or other equipment. _ : 

Modern burner design is carried out on the best scientific lines 
with a view to producing a burner satisfactory for all gas condi- 

8 


ILLUMINATING ENGINEERING 


218 
Adjustable gas checks, automatically mixing air supply, 


tions. 
properly proportioned Bunsen tubes and mixing chambers, a shapely 
exterior construction with the finest material and workmanship, 


make the modern burner a very effective and artistic appliance 


(Fig. 4). 


SY 


\ 
\ 
\ 
N 


oP 


<g D 


ae = wee 
re, 
3 0.2, 9, 
RQ 5 ‘s 
ex e% 





Fig. 5.—Claymond Inverted Lamp. 


Inverted Burners 
The most important step in the improvement of gas illumina- 
tion, embodying the use of the Welsbach mantle, has been the 


INCANDESCENT GAS MANTLES 219 


commercial introduction of the inverted incandescent lamp. The 
reasons which underlie the rapid development of this lamp appear 
to be improved efficiency, direct downward distribution of the light, 
decoratwe possibilities and more durable mantles. 

The first inverted incandescent lamps were made by Claymond 
and exhibited in 1882-1883 (Fig. 5). Considerable activity was 
subsequently shown by inventors, and numerous forms were ex- 
ploited without commercial success. With the advent of the Wels- 
bach upright mantle, this line of research was abandoned and no 
developments of any consequence were made for 8 years. 
Interest in the inverted light was renewed by the exhibition in 
Germany of a burner for the thorium-cerium mantle in 1900. 
These inverted lamps did not meet with marked commercial appli- 
cation in this country, because their designers failed to take into 
account the principles which modern inverted-burner builders recog- 
nize as basic. 

The history of the modern inverted burner is confined almost 
entirely to the development of methods of overcoming the comph- 
cated conditions of inverted combustion. 

Types. ‘Two general divisions may be made which involve dif- 
ferent applications of the principles of combustion. 

The first is based on a burner calculated to pre-heat the gas 
or air, or the mixture; and the other is a type where the con- 
struction is arranged to avoid, as far as possible, increasing the 
temperature of the gases before they reach the point of combustion. 

The advantage of pre-heating the gases before combustion is 
questionable, and prominent authorities may be quoted for and 
against the increased lighting efficiency to be obtained by this 
method. It might be inferred that the incandescence of the mantle 
would be increased by raising the initial temperature of the gases 
before entering the combustion chamber, but practical results show 
conclusively that the abnormal rarefaction of the gases due to the 
increased temperature of the mixture tends to produce the oppo- 
site effect. 

On the other hand, artificially cooling the gaseous mixture be- 
fore combustion produces a decrease in the efficiency. Further- 
more, the pre-heating or extreme cooling of the mixture complicates 
the burner construction. 

Inverted-burner designers are adopting the medium system, and 
are constructing a burner so that the gaseous mixture will main- 


220 ILLUMINATING ENGINEERING 


tain a temperature ranging between the extremely hot gases pro- 
duced in the regenerative type and the cold gases produced in the 
cooling type. 

In modern practice, inverted-burner construction falls under 
three general designs. 

First. The upright Bunsen, with the tube carrying the gas and 
air mixture curved through half the arc of a circle (Fig. 6). 

Second. The horizontal Bunsen tube, with the mixing chamber 
curved through one-quarter of an are (Fig. 7). 





Fig. 6.—Upright Bunsen Inverted Burner. 


Third. The vertical Bunsen, with a straight tube, for the de- 
livery of the mixture to the point of combustion (Fig. 8). 

Designers recognize as an essential feature of design and function 
the following general points: 

First. ‘The production of a proper mixture of gas and air under 
all conditions of operation, to insure perfect combustion. 

Second. Means for positively preventing flash-backs under all 
conditions of operation. 

Third. Special construction of the Bunsen tube designed to 
project the gas and air mixture downward to the point of com- 
bustion with maximum velocity, in order to overcome the ascend- 
ing tendency of the mixture. 


INCANDESCENT Gas MANTLES yved k 


Fourth. ‘The elimination of obstructions, long circuitous pas- 
sages for the mixture, or any other features offering frictional 
resistance to the projection of the gases toward the point of com- 
bustion. 

Fifth. The protection of the fresh-air supply from vitiation by 
the products of combustion. 

Siath. An efficient, well-constructed and reliable adjustable 
gas check. 

Seventh. Refractory construction at the burner head. 

Highth. Good mantles. 

Ninth. Glassware and reflectors specially selected and adapted 
for the effective and economical distribution of the light. 























Fic. 7.—Horizontal Bunsen Inverted Burner. 


Combustion, as applied to the Bunsen burner, must recognize 
three basic essentials: (a) the combustible, represented by the gas; 
(b) the supporter of combustion, represented by the oxygen of the 
air, and (c) the kindling temperature necessary to start the com- 
bustion, applied through the medium of a lighted match, electric 
spark, or some other heating means. Eliminate any one or more 
of these three essentials and combustion ceases. 

When a certain amount of gas is admitted to the mixing tube. 
of the Bunsen burner, a definite amount of oxygen (air containing 
oxygen) must be entrained and mixed with it in order to completely 
consume the combustible constituents of the gas. If the air supply 
is insufficient to meet these requirements, unconsumed or par- 
tially consumed constituents of the gas will be discharged from the 
burner, either in the form of solid particles of carbon or as noxious 


222 ILLUMINATING ENGINEERING 


gases. On the other hand, an excess in the supply of air results in 
a great reduction in the heating power, with its consequent de- 
crease of light, or produces a mixture below the critical point, which 
may result in a “ flash-back.” 





Fic. 8.—Vertical Bunsen Inverted Burner. 


The direct cause of a “ flash-back” is an explosive action which 
carries the flame into the mixing tube and sets up combustion at 
the gas orifice. The usual method used for overcoming the ten- 
dency to “ flash-back” is by placing a gauze in the burner tube 
at or near the point of combustion. This gauze serves to maintain 
the. mixed gases in the burner tube at a temperature somewhat 
below the kindling temperature, and thereby prevents combustion 


INCANDESCENT GAS MANTLES 220 


within the zone it protects. This will be recognized as the prin- 
ciple involved in the safety lamp invented by Sir Humphry Davy. 

The use of the gauze for preventing “ flash-backs ” is sometimes 
objected to on the ground that it obstructs and materially retards 
the downward projection of the mixture in the burner tube, and 
that it becomes clogged with dust and materially cuts off the 
mixture. 

The thermostat (Fig. 9) is a device placed in the lower Bunsen 
tube of one type of the inverted burner, and performs the function 
of a gauze without introducing its objectionable features. When 





Closed—Cold. Open—Hot. 
Fic. 9.—Thermostat. 


the lamp is cold the fingers of the thermostat are closed, forming 
a slitted cone which prevents a “ flash-back ” on lighting. As the 
lamp becomes heated the thermostat opens, leaving the Bunsen 
tube clear for the unobstructed flow of the gas and air mixture. It 
is so placed in the tube that it is not corroded by the action of the 
flame, and its automatic movements prevent it from collecting dust. 
This thermostat is made from a double metai in which each side 
possesses a different coefficient of expansion; for example, brass and 
iron. The brass is placed inside, and due to its own rapid expan- 
sion when heated causes the curved fingers of the thermostat to 
straighten out and lie against the walls of the Bunsen tube. On 
cooling, the brass contracts more than the iron and the fingers 
resume their original curved position, forming the slitted cone 
biti? .9 ): | 


224 ILLUMINATING HNGINEERING 

The Bunsen tube, even in its highly developed form, now used in 
upright burners fails in some essential features when applied to the 
inverted burner. In considering this problem, it should be noted 
that the ordinary illuminating gases are lighter than the air and 


possess a marked ascending tendency even at the normal tempera- 
ture. When considered in connection with the heated condition 


Gas adjustment. 


Air adjustment. 


Raceway. 


Thermostat. 





> 
OG, 


Be Maite : 
ee stig : 





Crown for holding 
glassware. 


Refractory 
burner tip. 


Fie, 10.—Inverted Burner Cut to Show Interior Construction. 


of the inverted mixing tube it is seen that this ascending tendency 
is thereby greatly increased. 

The method used for overcoming the ascending tendency of the 
mixture is to project it downward with sufficient velocity to carry 
it to the point of combustion without regard to the specific gravity. 

The only force available for projecting the mixture downward 
is that obtained from the velocity of the gas at the check orifice. 
When it is considered that in many cases the initial gas pressure 


INCANDESCENT GAS MANTLES 225 


is very low, thereby greatly reducing the available force, and also 
that a certain portion of this force must be given to entraining the 
air for the mixture, it is obvious that great importance attaches 
to this function of the inverted burner. 

To meet the conditions of varying composition and pressure, or 





Fie. 11.—Gas Arc Lamp. Upright for Inside Lighting. 


uniformly low pressure in the gas supply, a construction is re-- 
quired embodying all the features of a highly efficient projector 
for gases. This requires an adjustable check which will give the 
greatest jet velocity to the gas as it is admitted to the Bunsen; 
air ports properly placed to give the most efficient entraining 
capacity; a “raceway” of correct diameter and length to give 
the mixed gases the velocity necessary to carry them through the 
mixing chamber and to the point of combustion. 


226 ILLUMINATING ENGINEERING 


An analysis of the large variety of inverted burners on the 
market, in the light of these facts and principles, will show a 
number which do not conform to any specifications except cheapness. 

Rapid progress is being made, however, and standardization will 
ultimately be reached on a combination basis of efficiency, relia- 
bility, convenience, durability, pleasing appearance—all with a fair 
and reasonable cost. 





Fic. 12.—Gas Arc Lamp. Upright for Inside Lighting. 


Gas Are Lamps and Clusters 


Following the introduction of the upright burner, high candle- 
power unit requirements were met by forming a cluster of indi- 
vidual burners, with separate gas cocks and chimneys, gathered 
under a common reflector. These groups of burners were next 
simplified by the introduction of a cluster of burners controlled 
by a single gas cock and surrounded by a single globe to replace 
the individual chimneys. This design of lamp was called a gas 
arc lamp, and it met with success on account of its simplicity of 
construction and easy maintenance. 

The principal aims in the development of the gas arc lamp have 
been to produce a unit (Figs. 11 and 12): 


INCANDESCENT GAS MANTLES Gee 


First. With a concentrated source of light. 
Second. With high efficiency. 

Third. Simplicity of operation. 

Fourth. Minimum cost of maintenance. 


Fifth. Individual gas adjustment for each burner. 


No principles differing from those encountered in the individual 





Fie. 13.—Gas Are for Outside Lighting. 


burner were involved in the development of this upright are lamp, 
although some perplexing conditions were met with. 


It was found that in order to approximate the efficiency of 
the individual burner, the are would have to be constructed with a 
“stack ” to induce more active combustion at the burner heads. 
These stacks are made from fused enamel on steel, or from brass . 
in various finishes. Mechanical devices have been evolved for con- 
venient methods for renewing and replacing mantles, removing and 
cleaning glassware, and innumerable other methods of simplifying 
and economizing maintenance and up-keep. 

Upright ares have been successfully developed for use outside in 
places exposed to the action of wind and rain (Fig. 13). 


228 ILLUMINATING ENGINEERING 


Inverted Gas Arcs 
Arcs of the inverted type for both inside (Figs. 14 and 15) and 
outside (Figs. 16 and 17) lighting are just coming into use, and 
are being rapidly improved and developed with every prospect of 
great success. 





Fig. 14.—Inverted Gas Are for Inside Lighting. 


Two different methods of construction are utilized in the most 
prominent types of inverted arc lamps. One in which an indi- 
vidual Bunsen is provided for each mantle (Fig. 14), and the other 
where a single common Bunsen leads into a manifold head from 
which outlets are provided for each mantle (Fig. 15). Both of 
these types are now appearing in various sections, and experience 
alone will demonstrate the wisdom of the design. 


Incandescent Mantles 


The incandescent gas mantle was invented by Dr. Carl Auer von 
Welsbach in 1885 and 1886. 





INCANDESCENT GAS MANTLES 229 


The basis of Dr. Auer’s invention is the refractory hood or 
mantle made from an attenuated mixture of the oxide of thorium 
with a small percentage of cerium oxide. The cerium, which is 
present in quantities varying from 1% to 2 per cent, is not an acci- 
dental impurity as has been inferred, but is an essential constituent 
exerting, by very small variations in amount, a marked effect upon 
the candle-power and quality of the light. The candle-power-cerium 





Fic. 15.—Inverted Gas Arc for Inside Lighting. 


relation is best illustrated by the curve shown in Fig. 18, in which 
the candle-power is plotted vertically and the per cent of cerium 
horizontally. It will be noted that the maximum candle-power is: 
obtained with 1 per cent of cerium, and that a small amount of 
cerium more or less than 1 per cent causes the candle-power to fall 
off very rapidly. 

This peculiar result may be attributed to the existence at the 
1-per-cent point of a solid solution or a definite compound which 
possesses a higher emissivity than either the thorium alone or the 
cerium alone. 


230 ILLUMINATING ENGINEERING 


The manufacture of incandescent gas mantles is a most inter- 
esting and complicated chemical process, and by a peculiar coinci- 
dence resembles in the delicacy of the hand work involved the 
close attention to details and the technical supervision required in 
the manufacture of the incandescent electric lamps. 

A brief outline of the materials and processes involved in the 
mantle manufacture may be of interest. 





Fig. 16. 


The first step consists in knitting a tubular fabrie of open mesh 
from threads of some combustible organic substance which, after 
being properly saturated with the thorium solution, may be con- 
veniently burned out, leaving the ash in a more or less adherent 
mass reproducing the physical form of the original fiber. The 
selection and preparation of the original fiber is therefore a matter 
of vital importance. Imperfect fibers or threads, mineral impuri- 
ties, irregular knitting, etc., all directly affect the quality of the 
mantle. 

The present practice is to use threads made from natural cotton 
fiber, natural ramie fiber or artificial silk fiber. 


INCANDESCENT GAS MANTLES 931 





Fic. 17.—Inverted Arc Lamp for Inside Lighting. 


232 ILLUMINATING ENGINEERING 


The cotton thread must be made of a high grade, long staple, 
Sea Island fiber, uniform in size and free from knots or flaws. The 
tensile strength of the resulting mantle fiber depends largely upon 
the length and physical characteristics of the basic fiber. Further- 
more, if any knots, flaws, thin places, etc., exist in the threads they 
are reproduced in the mantle. 

Ramie is a natural vegetable fiber made from a substance known 
as “China grass.” The commercial supply of ramie is obtained 
almost entirely from China, India and Italy. In its crude form 
the ramie fiber contains large amounts of resins and mineral matter, 
and its purification is a very difficult and complicated chemical 
process. 


FREER EEE 
ZEBS 

Vf SA ee ee 
Pu NN ee ee 
fat | | ONE ae 
‘at ree 
PE a 
EEREREREESSST UU 
pe ee 
GEE 


Fia. 18. 














Ramie fibers are long compared with cotton and possess greater 
tensile strength and would naturally be expected to make a stronger 
mantle. While mantles made from a ramie base do not shrink as 
badly as mantles made from cotton their tensile strength is some- 
what disappointing, especially after being used for a time. 3 

Artificial silk, as the name implies, is an artificial fiber. It is 
made by dissolving cellulose in some suitable solvent to form a 
thick viscous solution, squirting this syrup through very fine dies, 
by great pressure, into some fixing bath. The resultant continuous 
filaments are then twisted into a thread. This thread may be 
knitted into mantle fabric and subjected to a special process of 
treatment for the production of a remarkably improved product. 
Mantles made from artificial fibers show improved physical 
strength, no tendency to shrink, no change in quality of light, 


INCANDESCENT GAS MANTLES 233 


and a remarkably small candle-power depreciation, even after 
several thousand hours of continuous burning. 

Saturating is a comparatively simple process, where the thor- 
oughly dried fabric is placed in a suitable vessel and covered with 
the lighting fluid. As soon as it is thoroughly saturated, the ex- 
cess of fluid is drawn off and the fabric is put through an equal- 
izing machine piece by piece, in order to bring each mantle to a 
uniform degree of saturation. 

In the highest grades of mantles the amount of lighting fluid 
used is based upon a careful consideration of the amount of oxide 
required to produce a mantle of the highest physical and candle- 
power life. 

The lighting fluid is composed of a solution of approximately 
99 per cent nitrate of thorium and 1 per cent nitrate of cerium in 
distilled water. This solution is usually of about 3 parts by weight 
of water to 1 part by weight of mixed nitrates. While the formula 
varies somewhat with different manufacturers, the limits are not 
wide. 

The commercial source of the nitrates of thorium and cerium 
is from a mineral known as monazite. This mineral occurs in 
commercial quantities only in North and South Carolina and in 
Brazil. The Carolinas’ monazite is found as a sand in the stream 
beds of the old mountainous districts, while in Brazil it occurs 
as a beach sand. | 

Monazite sand is mined on the principle involved in placer 
mining for gold. The gravel and associated minerals are shoveled 
onto screens and worked through into sluice boxes, where the min- 
erals of lower specific gravity are carried away by the water cur- 
rents, while the heavy monazite remains behind. This crude con- : 
centrate, carrying from 20 to 40 per cent monazite, is shipped to 
central plants, where it is further concentrated by the use of 
Wilfley tables and magnetic concentrators. The final product, as 
it is delivered to the manufacturer of lighting fluid, contains about 
90 per cent of monazite of the following average composition: 


PHGsonOric WMnNyY Aides; & cise ysin's.d ine < eas 28% 
Det aHE OS 10Gt. . . 8 cn padetem eae ck 30 
PEI ORIG o's oie sie ns eine ci wale se 14 
Neodymium and praseodymium ....... 16 
RNIN 408 1008 4s. So Se ea ee 5 
PeEM UT OX LGR! . Wa tS. Ga caddie eae 2 
Iron oxide, calcium oxide, etc.......... 5 
Matt ihen hom x cik wig Dare em CaM ad OMe ae es 100% 


234 ILLUMINATING ENGINEERING 


The manufacture of nitrate of thorium from monazite sand is 
a very difficult and complicated chemical process. It requires 
from 4 to 6 months to recover the small percentage of thorium and 
render it sufficiently pure to be used in the manufacture of lighting 
fluid. The by-products from this process have great scientific and 
chemical interest but no commercial value, and the thorium must 
stand the entire expense. The refined thorium nitrate must 
be chemically pure—free from all traces of mineral impurity and 
the other constituents of the monazite sand. 

The saturated fabric is now fixed for suspension by using as- 
bestos thread to form a loop, then shaped up preparatory to burn- 
ing out the organic material and converting the nitrates into oxides. 

The burning-out process is accomplished by igniting the fabric’ 
with a torch and waiting until the organic matter slowly oxidizes. 

After the fabric is completely consumed the ash of thorium and 
cerium oxides hangs in a soft, shapeless, flabby condition, and pre- 
sents a very remote resemblance to a mantle. 

When Dr. Auer first explained his idea for making a mantle 
to Professor Bunsen that famous teacher replied: “ It is extremely 
doubtful if the ash can be made to hold together.” This opinion 
was based upon Professor Bunsen’s knowledge of the general char- 
acteristics of metallic oxides, but the oxides with which Dr. Auer 
was working were notable exceptions. The incandescent gas light- 
ing industry depends upon this remarkable exception. 

After the organic matter is completely burned out in the process 
just described, the soft, flabby ash is carefully adjusted over a 
blow-pipe. The operator of this device controls levers which raise 
and lower the mantle, and which adjust the gas and the air supply 
to the blow-pipes. In some cases the gas is used under a pressure 
of several pounds to produce the intense flame required, but in 
either event the adjustment of the flame and the control of the 
position of the mantle is entirely in the hands of the operator. 

Under the influence of this intense blast flame the flabby ash, 
left when fhe organic fabric was burned out, is blown (by the 
proper control of the flame) into the required shape, and is changed 
from its soft, pliable state into a hard, resilient form. This opera- 
tion requires greater skill and experience than any other work con- 
nected with mantle manufacture. 

Coating. The object of this process is to form a protecting 
elastic skin over the ash to carry it while the mantle is going 





INCANDESCENT GAS MANTLES 235 


through the inspecting, trimming, packing, transportation and in- 
stallation stages. 

This coating, or collodion, as it is usually called, is made from 
soluble cotton. Soluble cotton is made by the so-called nitrating 
process in which the loose cellular fiber is treated with a mixture 
of sulphuric and nitric acids, and a product is formed closely 
allied to gun-cotton. 

This nitrated cotton, after being thoroughly washed and dried, 
is dissolved in some of the numerous solvents such as alcohol- 
ether, acetone, etc., and a thick, viscous liquid is produced. 

The collodion is placed in suitable vessels, over which the mantles 
are suspended and into which they are dipped, then transferred 
to hoods to dry. The mantles are then inspected and packed to 
meet the great variety of needs of the established markets. 

It is estimated that the American market consumes 60,000,000 
mantles per year, most of which are standard-sized upright and 
inverted mantles. Large quantities of mantles are also produced 
for railroad-coach lighting with Pintsch gas, kerosene lamps, gaso- 
line systems and high-pressure oil lamps. 

In the limited allotment of time for this subject, this review 
must necessarily be brief and superficial, but I have attempted to 
make it clear to you that the development, growth and future of 
the incandescent gas-lighting industry is a matter of immense 
scientific and economic interest. 


Core e ree 
ee ey er 
ye 7 





VI 


THE GENERATION AND DISTRIBUTION OF ELECTRIC- 
ITY WITH SPECIAL REFERENCE TO LIGHTING 


By Joun B. WHITEHEAD 


CONTENTS 
PRINCIPLES AND DESIGN 


1. Interior illumination. 

a. Systems of power supply: generating plants; constant potential; 
direct current, 2- and 3-wire; alternating current, 2- and 3-wire; 
alternating current, high voltage single and polyphase; trans: 
formers; isolated power plants. 

b. Systems of distribution: 2-, 3- and 5-wire parallel distribution, 
for incandescent glower, vapor and arc lighting; series parallel 
distribution; low voltage incandescent lamps on direct and 
alternating current circuits. 

c. Design of electrical system: Choice of system; regulation of sup- 
ply system; voltage drop in direct and alternating circuits; 
permissible voltage variation; size of feeders; diversity factor; 
number and sizes of branches. 

2. Exterior and street illumination. 

a. Systems of power supply: Constant potential and constant cur- 
rent, high and low voltage, direct and alternating; constant- 
current generator; constant-current regulators and rectifiers. 

b. Systems of distribution: Parallel and series parallel constant 
potential, for arcs and incandescents. Constant-current series 
systems for arcs and incandescents. Alternating current to 
direct current systems. ; 

c. Design of electrical system: Choice of system. Constant voltage 
and constant current regulation. Size of feeders. Power loss 
in series circuits; underground and overhead systems. 

3. Meters. 
a. Types of meter. 
b. Accuracy, calibration and inspection of meters. 


THe INSTALLATION OF ELECTRIC LIGHTING SYSTEMS 


1. Interior illumination. 
a. Type of installation. 'Two- and three-wire. Exposed and con- 
cealed wiring. Conduit systems and outlet boxes. 
b. Control. Service connections. Distributing centers. Switches. 
Protective devices. Subdivision of total copper. 
c. Relative costs. 


238 ILLUMINATING ENGINEERING 


d. Fire and insurance control. Ground connections. 
e. Specifications, drawings and contracts for work of installation, 
including materials. 
f. Tests. 
2. Exterior illumination. 
a. Type of installation, arc or incandescent, parallel or series. Over- 
head or underground systems. Insulation. 
b. Control. Automatic cut-outs. Protective devices, lighting 
arresters. 
c. Municipal restrictions. Underground construction and cables. 
2. Cost of operation. 
a. Cost of electric power. 
b. Systems of rates of sale of power; flat rates; maximum demand; 
two-rate systems. 
ce. Contracts for purchase of power. 


Principles and Design 


Electricity for lighting may be taken from any type of gen- 
erator. The earliest types of generator were developed to meet 
the requirements of lighting apparatus. With the introduction 
of other applications of electricity generators have been designed 
with characteristics to meet particular purposes, but it is probable 
that every operating generator furnishes more or less current for 
lighting. In modern installations, in which a large portion of the 
total capacity is consumed in lighting, the generators are designed 
with special reference to the regulation required by lighting ser- 
vice. Such generators are of various types, the extremes being 
the smallest continuous-current dynamo of the isolated plant for 
a single building, and the modern high-power (20,000 kw.) alter- 
nator of the city central station. | 

The proper circuit conditions for electric lighting are either 
constant potential or constant current. The general problem of 
central-station design to meet these conditions involves a knowl- 
edge of the various sources of energy, types of prime movers, gen- 
erators, control and regulating apparatus, and is distinctly within 
the province of the present-day electrical engineer. The electrical 
phase of the problem of the illuminating engineer will only in 
extremely rare instances contain the questions of prime movers, 
generators and station design. In general his concern, certainly 
in interior illumination, need go no further back than the avail- 
able service mains. At this point he need only recognize the type 
of service, know what regulation he may demand, and be able to 





GENERATION AND DISTRIBUTION OF ELECTRICITY 239 


draft a service contract for his client. From this point inward he 
must be able to design the distributing system electrically and 
mechanically, with due regard to fire hazard and conformity to 
local regulations. He must be able to draft a specification and 
prepare drawings for an installation which shall amply secure 
for his client a completed and tested lighting system for a definite 
price. ‘The exterior problem requires a somewhat wider knowl- 
edge of the principles of distribution, but will rarely, if ever, ap- 
proach the station nearer than, say, a constant-current regulator. 
In brief, the illuminating engineer can generally assume that the 
electrical engineer will furnish him with constant pressure or con- 
stant current. The electrical problem of the former is to know 
the limits of this constancy, and to be able to design, install and 
test the proper distributing system. Should the illuminating engi- 
neer ever desire to extend his knowledge to the engineering of 
generating equipment, many excellent treatises on the subject are 
readily available, and it does not appear desirable, in the short 
space allotted here to the electrical problem of the illuminating 
engineer, to devote more than occasional mention to a kindred 
topic of wide extent and well treated in the literature of the subject. 


1. Intertor Illumination 


(a) Systems of Power Supply. The commonest class of public 
power supply for interior lighting is at constant potential. In 
the hearts of cities it is usually in the form of continuous current 
supplied by an underground three-wire interconnected network 
of mains. This network is fed, over underground feeders con- 
nected to the mains at various points, from rotary converters or 
motor generators in one or more substations. The general plan 
of such a network is indicated in Fig. 1. These machines are 
operated by alternating current which is generated at voltages up 
to 15,000, or even higher, in central stations at some distance 
from the substation centers of distribution. The voltage of these 
networks is 220 to 240 between two so-called “ outside ” wires, and ~ 
110 to 120 volts between either outside wire and a third or 
“neutral” wire which is usually kept.at the potential of the 
earth, or “ grounded” by connecting to an underground system of 
water pipes, or by other methods. Most interior lighting devices 
are designed for voltages in the neighborhood of 110, and the aggre- 
gate load is divided as uniformly as possible between the two sides 


240 ILLUMINATING ENGINEERING 


of the three-wire network. In this way the two halves of the load 
are connected in series, and the distribution for 110-volt service 
is accomplished at 220 volts, with great saving in the amount of 
copper, since, at a given loss and distance, the amount of copper 
necessary varies inversely as the square of the voltage. The use 
of the neutral conductor, however, reduces the amount of this 
theoretical saving. The neutral conductor is made necessary by 
the facts that the component parts of the load on the two sides of 
the system are often separated by some distance, and especially 
that the two sides of the system are never exactly equally loaded. 





a ee Pee Ael, 
Main. 


Fig. 1. Fig. 2. 


Fic. 1.—Direct-Current Underground Network. 
Fig. 2.—Outlying Alternating-Current Distribution. 


The excess current of the more heavily loaded side flows back 
to the substation over the neutral conductors of the mains and 
feeders. This conductor therefore only carries the difference in 
the current of the two sides of the circuit, and in a large system 
with average balance of load between the two sides of from 2 to 
5 per cent, its cross-section may be considerably less, say one-half 
that of the outer wires. This system therefore requires a genera- 
tor connection at a point midway between the potentials of the 
outer terminals. 'This may be accomplished by operating two 
generators in series and connecting the neutral to their junction. 
By the use of various auxiliary devices single machines may be 
constructed for supplying three-wire service. 





._ GENERATION AND DISTRIBUTION OF ELECTRICITY 241 


The cross-section of the main conductors of such a network may 
aggregate several million circular mils, divided into lead-covered 
cables of 1,000,000 or 2,000,000 c.m. each. The feeder cables are 
usually somewhat smaller, with neutral one-half the section of 
the outside conductors. These feeders are provided with a small 
insulated strand leading back to the station, which serves to 
indicate in the station the potential at the network. The voltage 
drop in the feeders varies from time to time and may be as great 
as 10 per cent. 

In locations at sonie distance from the central station or sub- 


/IOV 

me eee 
3 | S/o V 
: S1ov 
: Slo VW 


Fig. 8. Fig. 4. 


Worke2 Vv 





Fie. 3.—Alternating-Current Secondary Network. 
Fic. 4.—Two-Phase Three- and Four-Wire Systems. 


station power is transmitted as high-voltage alternating current, 
and the voltage lowered by transformers which feed into the con- 
sumers’ circuits. For the extreme outlying districts with widely 
scattered consumers, each is often fed from a single small trans- 
former located at the property line, and supplying power over 
two wires only. In intermediate regions where the consumption 
is fairly dense several consumers may be fed from the same trans-. 
former, as indicated in Fig. 2. For still denser regions, beyond the 
reach of the continuous-current network, a secondary alternating- 
current network fed by several transformers at different points 1s 
_ sometimes formed (Fig. 3). In each of these cases the three-wire 
system is commonly used with 220 to 240 volts: on the outer wires, 
and the neutral connected to the middle point of the transformer 


242 ILLUMINATING ENGINEERING 


secondaries. Both the primary and secondary circuits in this 
class of supply are usually carried overhead, though not invariably. 

The modern large central station generates at 25 cycles, three- 
phase. This frequency is the best for transmission and for trans- 
formation to mechanical power. It is not, however, well adapted 
to either arc or incandescent lighting, although there are many 
instances in which it is used for the latter. Economy of trans- 
mission copper and the superiority of the polyphase motor for 
power service result in the general use of three-phase instead of 
single-phase primary circuits. 

For lighting from such systems motor generators are often used 
for changing from 25 to 60 cycles, the latter being the standard 
frequency for alternating-current lighting. 


Hiab. Fa. 6. 


Fic. 5.—Three-Phase Three-Wire System. 
Fic. 6.—Three-Phase Four-Wire System. 


The 60-cycle generator for city lighting operates usually at 
some voltage between 2200 and 2600. Since there is always a 
market for power also, it is commonly of two- or three-phase type. 
Secondary lighting circuits at 220 to 240 volts are obtained from 
the polyphase primaries by transformers connected in various ar- 
rangements. If the service is for lighting only, single-phase sec- 
ondaries only are needed, and single transformers for the separate 
loads are connected to the various phases, and in such manner 
that the aggregate load is as nearly as possible divided evenly 
among the several phases. In some instances, however, there is 
a power load requiring small motors which cannot be operated at 


the high primary voltage. These moderate-size motors are also . 


most satisfactory in the polyphase type. The secondary distrib- 
uting system must therefore be polyphase, and this is accomplished 





GENERATION AND DISTRIBUTION OF ELECTRICITY 243 


by various transformer combinations, resulting in three-, four- 
and even five-wire secondary systems. Figs. 4, 5, 6 and 7 show 
several of these combinations. Lighting circuits may be taken 
from any one of the branches of such a symmetrical system. This 
results, however, in placing both lamps and motors on the same 
transformer. Since the alternating-current motor often takes a 
starting current several times as great as its normal running cur- 
rent, the starting of the motors frequently results in a momentary 
fluctuation of voltage which is noticeable at the lamp. For the 
most satisfactory results the lighting and power loads should be 
on separate transformers, as the greater part of the voltage dis- 
turbance occurs in the secondary distributing circuit and in the 


transformer itself. 
: : //OV 


tig tenn Fie. 8. 


Fic. 7.—Two-Phase Five-Wire System. 
' Wie. 8.—Two-Wire Distribution. 


— 
. 
8 
NE 





An important type of supply system is the so-called isolated 
plant of a single large building or factory. These plants are either 
steam-, gas- or water-driven, and generate current of the class and 
voltage required at the lamp. Thus many of the earliest plants are 
equipped with 110-volt, two-wire continuous-current compound 
generators. Those of more modern design, however, have 220-volt 
three-wire generators, as the extent of the distributing system is. 
usually sufficient to demand the resulting economy in copper. This 
type of plant for lighting alone constitutes as reliable a source of 
supply as can be obtained. Properly chosen, the generating plant 
will give as constant voltage regulation as may be desired, and 
satisfactory performance of the lamp will then depend only on 
the design of the distributing system. Usually, however, these 


244. ILLUMINATING HNGINEERING 


plants must also furnish power. Little trouble is caused if the 
motors are of moderate size and the power load fairly constant. 
The elevator or other similar motor, however, with its large starting 
current will usually cause serious voltage fluctuations unless special 
provision is made for its fluctuating load. The most approved 
equipment for this purpose is the storage battery with “ booster ” 
generator; the battery automatically charges at values of load 
under the average and discharges when the elevator motors re- 
quire their greatest currents. This equipment also serves to equal- 
ize the demand on the generators between the light and heavy 
portions of the daily load curve, the battery charging during the 
morning hours and discharging as the lighting peak comes on. 


HOV 
22ZO0V 
SioVv 
S1OV 
R2O0V 
SLOV 


Fig. 9.—Three-Wire Distribution. 


(b) Systems of Connection. Except in rare instances all in- 
terior electric lighting is obtained from lamps designed for constant 
voltage. The range of this voltage between 110 and 130 volts | 
is that within which the incandescent lamp can be most satis- 
factorily constructed. This has probably been the most important 
influence in fixing this practically standard value for the voltage 
of low-tension distributing circuits. The various other types of 
lamp for interior service have, therefore, naturally developed with 
conformity to this voltage, or to double its value, as obtained from 
the outer wires of the three-wire distributing system. 

Incandescent lamps, therefore, for interior illumination may 
be fed from the simplest type of two-wire distribution, shown in 
Fig. 8, or they may be connected between either outer wire and 
the neutral of a three-wire system, as in Fig. 9, or they may be 





GENERATION AND DISTRIBUTION OF ELECTRICITY 245 


connected across any branch of a secondary polyphase lighting and 
power network, as already described. The usual voltage in these 
cases is between 110 and 130. Incandescent lamps for operation 
on 220 volts are obtainable, and are sometimes used on a 440-volt 
three-wire system to secure the benefits of the higher voltage for 
distribution. These lamps are inefficient and less rugged than 
those of lower voltage, and this system has not been generally 
adopted. 

Obviously the incandescent lamp may be supplied from either 
alternating or continuous-current circuits. Several exceptions to 
this statement must be noted, however. The life of the tantalum 
lamp, for reasons not yet understood, is shortened when used on 
alternating circuits; the amount of this shortening is about 50 
per cent when operated at 60 cycles. In many instances it is 
possible to detect a flicker in lamps operated from 25-cycle circuits ; 
this is most noticeable in lamps of low candle-power and high 
voltage in which the filament is necessarily of small diameter. 
Nernst lamps operate on 110- and 220-volt constant-potential 
alternating circuits. A glower adapted to continuous current has 
been developed but has not met with success. Such lamps, there- 
fore, are adapted to the several types of alternating-distributing 
systems only. 

The mercury vapor lamp is best adapted to constant-potential 
continuous-current systems, but is also manufactured for alter- 
nating service. It may be constructed for a wide range of voltage, 
but is commonly manufactured for 110- and 220-volt circuits. 

Interior illumination of stores, factories, etc., by means of arc 
lamps is quite common, and in such instances the lamps are oper- 
ated from constant-potential circuits. Although the are lamp in 
its best form is a constant-current device, constant-current cir- 
cuits are usually of a voltage too high for introduction into build- 
ings. Multiple or constant-potential arc lamps have, therefore, 
been developed, and it is now possible to secure an arc lamp suit- 
able for any type of available supply system. Thus single are 
lamps may be supplied from either 110-volt side or from the 
220-volt outer wires of either a continuous or an alternating 
three-wire distributing system. Lamps for 110 volts continuous 
current may be operated singly or two, four and five in series 
from 110-, 220-, 440-, 550- (railway) volt circuits. Lamps may 
be operated singly, by means of compensators or transformers from 
alternating circuits of any voltage or frequency. 


246 ILLUMINATING ENGINEERING 


Incandescent lamps are frequently connected several in series, 
where the available voltage is higher than that of the lamps. The 
most familiar instance of this method of connection is found in 
the cars and buildings of the street-railway system operating at 
550 volts. The introduction of the efficient metallic filament has 
led to an extension of this method of connection, as applied to low- 
voltage, low candle-power incandescent lamps operating from 110- 
volt circuits. This method of connection has arisen from the 


desire for a lamp of lower rating and more durable construction | 


than the 25-watt, 110-volt tungsten. Standard-base tungsten 
lamps may now be had for any voltage below 130, and lamps 
of 1.25-watt consumption in 10-, 15- and 20-watt sizes and at 


voltages from 10 volts upward are standard with manufacturers. 


The obvious objection to this series parallel method of connection 
is the fact that the failure of one lamp cuts out the others in 
series with it. These lamps are especially hardy, however, owing 
to their short, stout filaments, and when once in place in rigid 
sockets the plan is well adapted to long passageways and other 
areas requiring four or more units of low intensity without inde- 
pendent control. Four 28-volt, 10-watt lamps in series on a 115- 
volt circuit is a very satisfactory instance of this type of connection. 
Ten 10- or 12-volt, 5-watt lamps in series are sometimes used 
for sign lighting, but the arrangement is not satisfactory owing 
to the result consequent upon the failure of one lamp. 

Multiple operation and independent control of low-voltage lamps 
on existing multiple wiring is possible on alternating circuits by 
the use of transformers and “economy coils” or auto-trans- 
formers. With the use of the latter it is possible to operate the 
lamps in series, and the failure of one lamp will not affect the 
others. 

(c) Design of Electrical System. ‘The designer of interior il- 
lumination will rarely if ever find it necessary to extend the system 
of electrical conductors beyond the property line. In cities, for ex- 
ample, the source of supply will be either an underground continu- 
ous-current three-wire network with connections from a nearby 
manhole available at the building line, or an overhead secondary 
alternating-current line at a greater or less distance, or in many of 
the larger problems the power supply will be in or very near the 
building itself in the form of an isolated plant. 

Considering, first, the source of supply, the careful engineer 





GENERATION AND DISTRIBUTION OF ELECTRICITY 247 


will consider (a) its capacity, (b) its reliability, (c) its voltage 
regulation, and (d) its distance and the class of current, i. e., 
whether continuous or alternating current, and if the latter, its 
frequency and voltage. The capacity of the source of supply, in- 
cluding the transmission conductors up to the point from which 
the new lighting load is to be taken, must be sufficient to ensure 
satisfactory and continuous operation of all the loads upon it. 
Moreover, the application or removal of any individual load should 
be without disturbing effect on any of the others. The questions 
of capacity and reliability are not apt to arise in connection with 
an underground continuous-current network, or with an individual 
isolated plant constructed especially for the system under design. 
Also, in the commoner instances of alternating-current secondary- 
distributing systems, since the transformers are the property of 
the supply company, the question of capacity will be taken care 
of by them. The question of reliability in such systems assumes 
importance when the transformers are at the end of long feeders, 
particularly if the latter pass through open country for any dis- 
tance. Suburban lighting from the best class of supply company 
is often subject to interruption from line troubles due to wind, 
snow and sleet, and lightning. For large installations, where even 
a short interruption is to be avoided, these considerations may be 
sufficient to justify an isolated plant. Generally, however, occa- 
sional brief interruptions in localities where this class of service 
is the only one available can be tolerated, and the engineer need 
only satisfy himself that the overhead lines are of approved con- 
struction and protected by lightning arresters of design and loca- 
tion dictated by the best present-day knowledge of this imperfectly 
understood portion of the problem. A committee of the National 
Electric Light Association is now engaged in an effort to standard- 
ize the methods of installing the various types of distributing 
system. At this time definite recommendations have covered sec- 
ondary-distributing systems only, and are contained in a report to 
the Association at its convention in May, 1910. ‘The work of this 
committee when completed will furnish an excellent reference for 
all questions of overhead lighting wiring. Should the engineer 
find it necessary to specify and install his own transformers, the 
questions as to the methods of installing them, together with their 
fuse blocks, are considered at length in the report mentioned 
above. The transformer capacity and subdivision, in its relation 


248 ILLUMINATING ENGINEERING 


to the installation, depend on the time distribution and concen- 
tration of the connected load. ‘These two elements are usually 
combined in the “ diversity factor,” or the ratio of the sum of 
the maximum demands of the several consumers to the maximum 
demand which actually results from their combined service. For 
transformers in residence lighting this factor is about 3, in com- 
mercial lighting from 1.6 to 1.1. 

Speaking generally, transformers may be operated for an hour 
or two at 50 per cent over their rated capacity, and for short 
intervals at 75 per cent or 100 per cent. On account of the short 
distances to which low-voltage alternating current may be trans- 
mitted, transformers on poles rarely exceed 15 kw. in capacity, 
and 30 kw. is about the limit in size for transformers for lighting 
only. 

The voltage regulation of the supply system, next to constancy 
of service, is the most important factor for satisfactory lighting. 
Too often the engineer has to be content in this particular with 
what he can get. In the present state of the art it is rarely pos- 
sible to secure from a supply company any statement or guarantee 
as to the limits of fluctuation of its voltage. Probably the most 
constant voltage obtainable is that in the best type of isolated 
continuous-current plant, as found in a few modern office buildings 
with special provision for motor loads. In this case the feeders 
are all short, and the regulation approximates the practical con- 
stancy obtainable in compound generators.. Often, however, the 


isolated plant has too little capacity, and carries both motor and 


lamp loads without special regulating apparatus. In such cases 
the regulation is very poor. 

The underground 220-volt, three-wire continuous-current net- 
work of the best type of city plant yields excellent regulation. 
Such a system comprises a close network of mains, often compris- 
ing several 1,000,000 circular-mils cables. The voltage in this 
network is maintained constant by connecting it at various points 
with feeders from the station. Potential wires, as already men- 
tioned, are also run to the station and indicate the voltage through- 
out the network. The voltage on the feeders is varied according 
to the needs by connection to several sets of bus-bars of different 
voltages, operated from separate machines, or through boosters 
and other regulating devices. The method is indicated in Fig. 10. 
The load changes of such a system, owing to its size, are quite 


. 
A 
q 
: 
; 
5 
j 





GENERATION AND DISTRIBUTION OF ELECTRICITY 249 


uniform, so that voltage adjustment at any point of the network 
is simple. In such a system a daily constancy within 1 or 2 volts 
is obtainable. 

Alternating-current secondary-distributing systems do not, as 
a usual thing, afford as satisfactory voltage regulation as the con- 
tinuous-current system. The drop in the primary wires is rarely 
a disturbing factor, since this is compensated for in the station, 
and that in the transformer may be less than 2 per cent on non- 
inductive incandescent-lamp load. A serious drop due to induc- 
tive reactance, however, occurs in the low-voltage secondary cir- 
cuits, and limits their length to comparatively short distances. 
For this reason secondary networks, commensurate in size with 
those of the continuous-current system, are not used. In such . 

iain 


oer 


111 {| 1] 
Fig. 10.—Station Connections Direct-Current Feeders. - 


an alternating-current network a transformer fed from a separate 
pair of primary wires constitutes a feeder corresponding to that 
of the continuous-current network, and since alternating-voltage 
regulation is simpler, the station apparatus of this system is less 
elaborate and, therefore, cheaper. ‘The transformers must be close 
together, however, owing to the drop in the secondary circuits, 
and this condition is greatly aggravated in the fairly common 
event of one transformer getting into trouble. These facts are 
sufficient to have restricted the alternating network to compara- 
tively limited areas. When several transformers are connected to 
the same primary circuit, station control compensates for the 
variable drop of changing load; obviously that this arrangement 
be satisfactory to all consumers, they should all have approximately 
the same type of load variation. It will be thus seen that the 
voltage regulation of alternating sources of supply may be good 


250 ILLUMINATING ENGINEERING 


or bad depending on the type of control at the station, the number 
of consumers on a line, the particular way in which these con- 
sumers vary their demand, etc. If the lighting circuits are also 
used for motors, it is still more difficult to secure good regulation. 
In the better classes of service momentary variations of 1 or 2 
volts in 110 should not be cause for complaint. As the load goes 
on the voltage is raised at the station either automatically or by 
hand, and this may cause an extreme daily variation of 3 or 4 
volts. Departing from the best class of service, it is possible to 
find almost any degree of poor regulation in lighting circuits. 
In these days, however, a total daily variation greater than 5 per 
cent should not be tolerated from a company professing to give 
first-class service. 

With the voltage variation of the supply system given, the engi- 
neer must design his distributing system so as to add as little 
voltage variation as possible, and so keep the voltage at the lamp 
as nearly constant as the source of supply will permit. The prin- 
ciples involved in this design are simple, and the problem is 
usually the very indefinite one of a decision as to what additional 
drop to allow in order to secure a low cost of the distributing 
system. With continuous current the application of Ohm’s law in 
one of its several forms will determine the size of conductor for 
the chosen voltage drop. ‘Temperature variation of resistance, 
however, must be duly considered. If many calculations are to 
be made it is usually worth while to make use of tables giving 
relations between current, voltage drop, distance, ete., such as may 


be found in Hering’s Wiring Computer and other like works. In. 


most cases, however, it will be more satisfactory to make calcula- 


tion using resistance tables with temperature factors clearly given. 


The loss or drop in voltage in alternating-current circuits is 
due to resistance and reactance. The resistance may usually be 
that given by any wire table with temperature correction. ‘The 
reactance drop is caused by the electromotive force induced in the 
circuit by its own alternating magnetic field. This electromotive 
force is therefore proportional to the current, and to the frequency, 
and to the self-inductance, which depends on the length, the sepa- 
ration and the size of the conductors. The mathematical expres- 
sion for the reactance in ohms is 27NL, and for the reactance 
volts 27NLi, N being the frequency, L the self-inductance and i 


the current. The resistance and reactance volts are both propor-_ 


GENERATION AND DISTRIBUTION OF ELECTRICITY 251 


tional to the current, but differ in phase by one-quarter of a 
period so that the total drop in volts is the square root of the 
sum of the squares of the resistance and reactance volts. While 
- the resistance decreases rapidly with increasing size of wire the 
reactance decreases very slightly, consequently there is in alter- 
nating-current distribution an early limit to the improvement 
in voltage regulation by increase in the size of conductor. It is 
for this reason that low-tension alternating circuits must be short. 
The resistance and reactance at 60 cycles per mile of a circuit of 
two No. 5 wires, 24 inches apart, in ohms, are 3.24 and 1.4; for 
No. 00 the values are .8 and 1.24; it is seen that for sizes in this 
neighborhood little is gained by increasing the size of wire. Com- 
plete tables of resistance, reactance and impedance volts for vari- 
ous sizes of wire, separation, frequency, are now readily available, 
so that calculations may be quickly made. Attention to the re- 
actance drop is especially necessary in designing overhead service 
connections with space separation, and must not be lost sight of 
even in interior wiring where the two sides of the circuit are 
close together inside one conduit. For example, two No. 0 wires 
in a 2-inch conduit may easily have an average interaxial separa- 
tion of 1 inch; the reactance per 1000 feet is about .1 ohm or, 
one-half as great as the resistance; the impedance is therefore 
.224 or 25 per cent greater than the resistance. 

Secondary-distributing networks must therefore have trans- 
formers connected at fairly frequent intervals. A common method 
is to run three-phase primaries, supplying three or four city blocks 
from one phase through three transformers with their secondaries 
connected to a common three-wire main. The next three blocks go 
on the next phase, etc., preserving the balance as far as possible. 
Speaking broadly high-class secondary distributing or service cir- 
cuits should not exceed 400 to 600 feet in length, or between 
transformers. 

In calculating the wiring for any installation two types of volt- 
age drop must be considered, viz., that due to the gradual daily 
increase of the total load, and that due to the sudden cutting in 
or out of a portion of the total load. The former occurs gradually 
and principally in the mains from the supply system, and in the 
“risers” and distributing feeders. If the maximum load on all 
branches is definitely known, the voltage drop due to this cause 
may, of course, be kept within any limits by proper choice of con- 


252 ILLUMINATING ENGINEERING 


ductors. A wide range of variation of this kind is very objection- 
able. If the lamps are chosen for the high voltage, their luminous 
efficiency is impaired as the load goes on; if for the low voltage 
the life of the lamps used at light loads is shortened. As this 
type of variation is gradual it is not noticeable, and too little at- 
tention is given it in design. The second type of voltage variation 
is necessarily less in amount than the first, and is principally ob- 
jectionable in causing a momentary fluctuation of light from other 
burning lamps. This disturbance is reduced by designing so that 
the smaller part of the total permissible drop takes place in the 
mains and principal distributing feeders, and by increasing the 
number of branch feeders. A change of 1 per cent in the voltage 
on a tungsten lamp causes a variation of 4 per cent in its candle- 
power. Fluctuations of this nature, therefore, are to be par- 
ticularly guarded against in those cases where there is frequent 
cutting in or out of large numbers of lights. 

It is difficult and scarcely necessary to fix an absolute limit to 
the permissible voltage variation on an incandescent lamp. Sat- 
isfactory illumination is given by the 120-volt tungsten lamp over 
a range of 4 volts or more than 3 per cent; in fact, a given lamp 
is now rated for 3 voltages covering this range. The important 
consequences of this variation are the effects on the efficiency and 
life of the lamp, rather than on the illumination. Speaking gen- 
erally, with a supply system constant to within 1 or 2 per cent, 
very satisfactory service will be given if the maximum voltage 
drop inside the service connection be limited to 3 per cent. Of 
this the smaller part should be in the service wires and larger 
branches. A greater drop than this may be allowed when the 
greater proportion of the lamps are operated together, and so 
cause approximately a fixed drop in the service wires. With the 
entire load connected as one unit, i. e., with no independent opera- 
tion of single lamps, any amount of drop in the service connection 
may be allowed, by a proper choice of lamp. The calculation of 
the size of service wires, feeders and branches to meet the require- 
ments of voltage is thus a simple matter of distances as soon as the 
location of the individual outlets and sizes of lamps are fixed. 

The usual installation begins at the service wires, which may 
be either overhead or underground; these are generally 220-volt, 
three-wire, carried directly to a center of distribution, which may 
be of any degree of elaboration. A simple iron box containing 





GENERATION AND DIsTRIBUTION oF ELECTRICITY 200 


a main switch and fused branch cut-outs suffices for a small resi- 
dence. Tor large installations a switchboard having panels for 
the main connection and individual feeders, as found in the 
largest buildings, may be required. From this center feeders 
run to distributing centers in various portions of the building. 
From these distributing centers sub-feeders are often taken to 
smaller local centers, though, more commonly, so-called branch 
circuits lead directly to the lamp outlets. The number of feeders 
and sub-feeders is regulated by the height and the floor area of 
the building. For great heights individual feeders for one or 
more floors may be necessary. Generally, however, several floors 
may be fed from one riser. For large areas, sub-feeders from the 
distributing to local centers may often be used to advantage. The 
three-wire system, is carried to the centers where branch circuits 
are connected. Probably the most important factor in determin- 
ing the number of feeders is the permissible length of branch 
circuit. The “ National Electrical Code” limits the lamp capacity 
of a single branch circuit to 660 watts. This figure was chosen 
as representing twelve 55-watt carbon lamps. Under this rule 
it is now possible to install twenty-six 25-watt tungsten lamps, 
although such a plan is not advisable. Further, no wire smaller 
than a No. 12 B. & 8. should be used for the branch circuits. At 
115 volts, 660 watts represent about 5.5 amperes, and the re- 
sistance of No. 12 wire is 1.62 ohms per 1000 feet. An average 
length of 50 feet of this circuit would therefore cause a drop of 
1 volt. With good regulation of supply system and ample copper 
in service wires and feeders, branch circuits may sometimes have 
a length of 100 feet, but this should be the maximum of conserva- 
tive practice. Exceptional cases may be met by increasing the 
size of the branch circuit. A radius between 50 and 100 feet, 
therefore, marks the area to be fed from one center or feeder con- 
nection. In small buildings, such as residences, therefore, no 
feeders are required, all branch circuits starting from a suitable 
distributing board where the service wires enter. In larger build-_ 
ings the density of the load on various floors will determine 
whether more than one floor may be fed from one feeder. The 
low consumption of tungsten lamps will usually permit two or 
three floors to a feeder or a riser, with 10 or 12 branch circuits 
to a floor. In such a case it is advisable to place a distributing 
board on each floor, and not extend the branch circuits from a 
center on one floor to outlets on floors above and below. 


254 ILLUMINATING ENGINEERING 


With all feeders brought back to the service connection, which 
should be located as near the mean center of load as possible, it is 
a simple matter to calculate the voltage variation on any lamp 
with all lamps burning. With a fixed limit of variation, this 
condition will usually call for larger feeders-and service wires than 
necessary. A study of the particular problem only can determine 
the probable maximum number of lights to be operated at one 
time. For residences this number will rarely exceed one-third to 
one-half of the total, while in office buildings, churches, theaters, 
etc., the total connected load may often be in operation at one 
time. It is only in exceptional cases that the cost of copper in 
the feeders is a sufficiently large proportion of the total cost to 
warrant the reduction of their size. No great increase in cost 
will generally result from designing service wires and feeders to 
the end that the operation of the maximum connected load will 
only cause the permissible voltage variation on the lamp most un- 
favorably located. 


2. Exterior and Street Illumination 


(a) Systems of Supply. Exterior illumination may be taken 
from any available source of supply. The use of constant-poten- 
tial continuous-current service is limited to loads concentrated 
within a small area, owing to the fact that the distributing 
losses mount very rapidly for any considerable distance. Instances 
of this type of supply are electric signs from 110- to 220-volt 
mains, multiple are lamps in front of buildings or stores, and 
street arches supplied from 550-volt railway circuits, the lamps 
being connected in series-parallel. 

Constant-potential alternating current at 2200, 4400 or 6600 
volts is probably the most common type of exterior supply circuit, 
and it may be utilized in various ways for exterior lighting. It 
may be simply transformed to low-voltage, constant-potential ser- 
vice or to high-voltage, constant-alternating current for series are 
and incandescent circuits, or to high-voltage continuous current 
by means of the mercury rectifier. The last mentioned is perhaps 
the most satisfactory of all methods of are lighting. 

For many years constant-continuous current, series arc circuits 
were supplied from Thomson-Houston and Brush constant-current 
generators. Many instances of the latter type of installation are 
still in operation, and these machines operate with as high voltage 


1 ON ns Ot Pe a eee ee cea = 


GENERATION AND DISTRIBUTION OF ELECTRICITY 255 


as 13,000 with currents of 5 to 10 amperes supplying upwards 
of 220 lamps. These excellent machines, after a highly honorable 
record, are now being rapidly supplanted by constant-potential to 
constant-current transformers fed from 2200 volts constant-poten- 
tial alternating circuits and supplying on the secondary side con- 
stant-alternating current. These transformers are equipped with 
one stationary coil and a movable coil which automatically shifts 
its distance from the fixed coil to meet the demands of the load. 





Fic. 11.—Constant-Current Transformer. 


In a transformer under load there is a repulsive force between 
the two coils. In ordinary constant-potential transformers this 
force is held in check by the close-fitting iron of the magnetic 
circuit. In the constant-current transformer this force is allowed. 
to act, free motion of the secondary away from the primary being 
allowed by providing a greater opening in the magnetic circuit 
than is required by the cross-section of the coils. But a separation 
of the two coils, due to a rise in current, is accompanied by a fall 
in the secondary voltage, since a portion of the magnetic field set 
up by the primary leaks across the gap between the coils and so 


256 ILLUMINATING ENGINEERING 


does not pass through the secondary. The tendency to a rise in 
current is thus checked by a fall in voltage. By means of suitable 
counter-balancing of the weight of the movable coil, and by other 
auxiliary devices, the transformer regulates very closely for con- 
stant current, and arc circuits may be taken directly from their 
secondaries. 

Fig. 11 shows a picture of this transformer. More satisfactory, 
however, is the series continuous current are circuit, which may 
be-had by combining with the constant-current transformer a 
mercury-are rectifier. ‘The combination gives excellent constant 


A C Supply 


Regulating 
Transformer 


Mercury 
Rectifier 


Starting 


Transformer 





D C Circuit . 
Fig. 12.—Direct-Current Series Arc Rectifier. 


continuous current regulation. The method of connection is il- 
lustrated in Fig. 12, and the apparatus provides a very reliable 
means of transformation between constant alternating-potential 
and constant continuous current. These equipments are available 
for any voltage between 220 and 13,000, and for any standard of 
frequency. ‘They may be had in sizes supplying as many as 75 
lamps. 

(b) Systems of Distribution. Arc lamps may be operated from 
any available source. Their operation on low-voltage, constant- 
potential circuits is less satisfactory than on a constant-current 
circuit. ‘The alternating-current multiple lamp is the most un- 
satisfactory of all, but its use is often justifiable in outlying dis- 





GENERATION AND DISTRIBUTION OF ELECTRICITY aod 


tricts with widely scattered lights. The constant-current series 
method of connecting arc lamps is the most common of all, and 
the series circuits may be either alternating or continuous current 
with preference for the latter. These circuits cover wide expanses 
of territory, and since the connection from lamp to lamp is by one 
conductor only the distributing system is simple and may be 
looped in various directions. 

Exterior lighting by incandescent lamps may also be adapted 
to any class of service. For condensed loads, such as signs, it is 
possible to use the multiple connection from low-voltage circuits 
using standard lamps. Low-power, low-voltage lamps must be con- 
nected two or more in series on continuous-current circuits; on 
alternating circuits the use of small transformers or economy coils 
will avoid the undesirable series connection. Street lighting is 
sometimes accomplished by the series-parallel connection of in- 
candescent lamps on railway circuits or other available constant- 
potential lines, but the most acceptable system for distributed 
and uniform lighting by incandescent lamps is the series connection 
of a number of these lamps across constant-potential high-voltage 
alternating circuits, or in constant-current circuits, either alter- 
nating or continuous. On constant-potential high-voltage service 
the lamps are shunted by small reactance coils, so that the circuit 
is continuous, and the voltage consumed by reactance if an indi- 
vidual lamp should fail. Tungsten lamps for this system are to 
be had with ratings of 1.75 to 4 amperes, and from 8 to 40 volts, 
so that it is possible to operate 260 such lamps in series on 2200- 
volt circuits. It is claimed for the system that operation is still 
satisfactory with 20 per cent of the lamps broken or out. The 
series connection of tungsten lamps may also be operated at 
constant-alternating current by use of the constant-current trans- 
former already described. By this method upwards of seven hun- 
dred 30-watt, 3.5 or 6.6 amperes, lamps may be operated in series. 
In this system each lamp is equipped with an insulating film which 
withstands the lamp voltage but breaks down if the lamp fails, thus 
preserving the continuity of the circuit. The transformer ad- 
justs automatically for the lowered voltage. 

(c) Design of the Electric System. Constant-potential regula- 
tion is obviously much less important in. outside than in inside 
lighting. Incandescent lamps employed in this class of service are 
comparatively few, and the objections to voltage fluctuations are 


258 ILLUMINATING ENGINEERING 


practically limited to the effect on the life of the lamps. The arc 
lamp is essentially a constant-current device and is not seriously 
affected by slight voltage variations. For the short connections 
usual in the use of constant-potential are lamps no special caleula- 
tion is necessary for the wiring beyond providing ample current- 
carrying capacity. Series arc lamps take from 4 to 914 amperes, 
according to the type. The commonest types are the 4-ampere 
continuous luminous lamp and the 6.6-ampere alternating or con- 
tinuous enclosed lamps. The regulation of the Brush generator, 
of the constant-current transformer, and of the mercury rectifier 
are all extremely close; it follows, therefore, that the design of 
the distributing system for exterior illumination will very rarely 
involve any serious problems of voltage regulation. The series 
circuits themselves and the resistance in lamps consume a large 
part of the applied voltage, and the constant-current regulating 
devices adjust automatically to a wide range of resistance. The 
series circuits of large cities carry 50, 75 and 100 lamps at volt- 
ages from 4000 to 8000. The distance of the separation of lamps 
varies, but averages from 200 to 300 feet. ‘The voltage drop in 
the conductor itself is usually between 5 and 10 per cent, and 
wires in the neighborhood of No. 8 B. & S. are used. The low- 
tensile strength of smaller wires renders their use inadvisable. It 
is not uncommon to find a circuit of this kind comprising 10 miles 
of single No. 8 wire and seventy-five 4-ampere lamps. 

Series incandescent lighting from constant-potential high-volt- 
age circuits is accomplished with lamps taking from 1.5 to 4 
amperes. For voltages above 550, the circuit should be insulated 
from the main line by a transformer. In series incandescent cir- 
cuits fed from constant-current transformers the lamps may be 
had for currents between 1.75 and 10 amperes. The common size 
of wire for this class of service is from No. 10 B. & 8. up. The 
regulators are rated in terms of the aggregate kilowatt capacity 
of total connected lamps. This rating includes an allowance of 
5 per cent ohmic and 10 per cent reactive drop in the series 
circuit. | 

The insulation of overhead conductors should be of the best 
rubber core and braided class of manufactured product. The 
underground conductor may be either fiber-, paper- or rubber-in- 
sulated stranded conductor, and in every case is surrounded by 
lead. These cables should withstand the test prescribed for all 


GENERATION AND DISTRIBUTION OF ELECTRICITY 259 


high-voltage apparatus, namely, they should be subjected to double 
the maximum voltage for a period of 1 minute. 


8. Metering 


The subject of metering is a highly important one in the com- 
plete discussion of the entire electric-lighting system. Meters are 
usually owned, inspected, tested and read by the company supplying 
power. The illuminating engineer will rarely be called upon to 
do more than provide proper spacing and accommodation for 
meters. 

Almost invariably the present-day meter measures watt hours. 
For continuous-current service the best meters are essentially the 
same as the original Thomson watt-hour meter. This consists of 
a continuous-current shunt motor containing no iron. The line 
current flows in the field circuit and the line voltage is applied 
to the rotating armature with the insertion of a very high re- 
sistance. This permanent shunt connection across the circuit, 
therefore, takes current at all times. While the current of an 
individual meter is extremely small, nevertheless, the aggregate of 
the meters of a large system results in quite an appreciable fraction 
of the total load on the station. The retarding force on the arma- 
ture of this meter is a copper disc rotating between the poles of 
several permanent magnets. The shaft of the armature is equipped 
with a small pinion which engages a train of gears connected with 
dials constituting the recording mechanism. For alternating cur- 
rents the induction-watt meter has many advantages over the 
Thomson type, although the latter may be adapted to alternating- 
current service. Induction meters operate on the induction-motor 
principle, series and shunt coils with different phase character- 
istics, giving the two components of the rotating magnetic field. 
A light aluminum disc constitutes the rotating element or arma- 
ture. By their principle they read true power, and are independent 
of phase difference between current and electromotive force. 

Alternating-current meters are, in general, more permanent and | 
reliable than those for continuous currents, in that they have no 
commutator nor brushes. The present-day meter, as furnished by 
the best manufacturers, has been brought to a high degree of 
perfection, and may be relied on to a very close figure of accuracy. 
No meter, however, will maintain its calibration indefinitely, and 
those in service should be tested and inspected regularly. This 


260 ILLUMINATING ENGINEERING 


is generally carried out by the supply company, which in most 
cases owns the meter. In some instances a meter rental is charged 
for the purpose of covering not only the original cost of the meter 
but for defraying this regular charge for inspection and repair. 
Questions sometimes arise between customers and supply com- 


panies as to the accuracy of meter readings, and public service com- — 


missions have in many places provided regulations by which a 
consumer may demand a test and calibration of his meter at any 
time by the payment of a small fee. The usual method of testing 
meters is that of comparing them with portable standard meters. 
It is, of course, necessary that these portable meters should be 
compared with permanent standard instruments in the laboratory 
at sufficiently frequent intervals. The methods of charging for 
power for lighting as based on meter readings will be referred to 
later in these lectures. 

The general subject of meters has been exhaustively covered by 
the reports of the committee on meters of the National Electric 
Light Association for 1909 and 1910. 


Tre INSTALLATION OF ELEctrRic LIGHTING SYSTEMS 


1. Interior Illumination 


(a) Type of Installation. The engineering questions arising 
in connection with the installation of a system of electrical con- 
ductors for distributing electric power for lighting are compara- 
tively simple. Such distribution is accomplished at moderate volt- 
ages for which the space requirements are not great. The usual 
problem is that of running a more or less elaborate system of two- 
or three-wire circuits inside a building. The objects which must 
be had prominently in view are those of safety, reliability, per- 
manence and unobtrusive appearance. ‘The system must operate 
without danger of fire or to life, The possibility of fire arises in 
the results following short-circuits and grounds in the system. 
The danger to life is not generally present in continuous-current 
service but arises in alternating-current distributing systems fed 
from transformers supplied by high-potential primary circuits. It 
is obvious that satisfactory operation will require that at all times 
the system will perform its functions of not only distributing 
power, but in permitting its ready control and the prompt elimi- 
nation of all abnormal conditions likely to cause interruptions. 


i i a 


GENERATION AND DISTRIBUTION OF ELECTRICITY 261 


The life of the installation depends largely on the materials and 
quality of labor entering into its construction. In this regard 
possible exceptions may enter in the installation of systems which 
are to have intentionally a short existence. Generally speaking, 
however, the material and workmanship of electric-lighting in- 
stallations should be of the best obtainable, and in accordance with 
the latest recommendations of engineering bodies. The distrib- 
uting system for residences, hotels and dwellings, generally, as 
well as in all buildings where agreeable and attractive appearance 
is required, should be as unobtrusive as possible. This considera- 
tion in the instances mentioned leads to the entire concealment 
of electric wiring. In factories and other buildings where no 
particular attention is required as to appearance, the conductors 
and supports are often installed exposed. This method is a per- 
fectly satisfactory one, if due attention is paid to the location of 
the conductors in such places as will render them free from 
mechanical injury. Exposed wiring presents the general advan- 
tage of accessibility and convenience of inspection. Concealed 
wiring, on the other hand, is almost invariably free from the 
danger of mechanical injury. Decision as to which general method 
should be followed will depend on the particular conditions of the 
problem. 

The methods of installing electric wiring are rigidly controlled 
by the National Board of Fire Underwriters, and the regulations 
governing this class of work are published by that body in a pam- 
phlet known as “The National Electrical Code.” In addition to 
these rules there is published a list of manufactured material which 
has been subjected to laboratory test, and which is known briefly 
as “approved ” material. In many cities there is a further list of 
requirements which apply to particular local conditions., 

_ Four classes of interior wiring are usually permitted. They are 
known as “ open-work,” “ moulding,” “ concealed-knob-and-tube ” 
and “ metal-conduit ” installations. In open work the wires are 
run entirely exposed and supported on porcelain insulators and | 
knobs; they pass through all walls, joist, partitions, etc., in por- 
celain tubes. The space requirements in the way of separation of 
wires from each other, and from walls and their relation to other 
circuits, etc., are rigidly specified. This type of installation is 
entirely satisfactory where its appearance can be tolerated, and 
is the simplest and cheapest to install. The principal precaution 


262 ILLUMINATING ENGINEERING 


to be taken is against mechanical injury. Moulding and knob and 
tube work have been developed as methods for installing wiring in 
buildings originally constructed without any idea of future elec- 
tric service. They represent the most unreliable and unsatisfactory 
types of wiring installation. In the case of moulding the wires 
are run behind either wood or metal strips which are laid on the 
ceilings and walls of interiors. In knob and tube work the wires 
are concealed by “ fishing”? them from point to point behind the 
plastering and under the floors of buildings without disturbance 
to these surfaces. This method is highly undesirable, and even 
when most carefully installed during the progress of building in- 
troduces great danger of fire. Both moulding and knob and tube 
work are make-shifts, and should never be installed by a careful 
engineer unless absolutely unavoidable. 

The complete enclosure of the entire wiring system up to the 
lamp or fixture outlet in metal conduit represents the best present- 
day method, and one which bids fair to form the ultimate standard 
of construction. In this system the entire wiring is completely 
surrounded by metal. The materials are to be had in the form 
of rigid or flexible metal conduit. The rigid conduit consists of 
iron pipe of various sizes, and usually in 10-foot lengths. Elbows, 
bushings and other fittings are also supplied for each size. This 
conduit is usually made as soft as possible to permit easy bending 
for adaptation to building peculiarities. It is either galvanized 
or covered inside and out with some protective enamel which is 
valuable in protecting the metal of the conduit rather than as 
insulation to the conductors enclosed. Flexible conduit com- 
prises the several varieties of the familiar tubing made in spiral 
form from cut steel. This conduit is best adapted to locations 
where straight runs are few, and where there is difficulty of access 
to wiring compartments. With either system of conduit construc- 
tion iron boxes are used for all classes of outlets. The conduit 
leads to these boxes and is mechanically and electrically connected 
to them by means of washers and nuts.. These boxes form con- 
venient points for pulling the wires into the conduit after the 
latter is installed, and also for making connections for branch 
circuits. This type of installation is readily installed in new 
buildings, whether they be frame, brick, concrete or other class 
of construction. Old buildings may generally be equipped with 
electric wiring in flexible-steel conduit with permanent damage 


GENERATION AND DISTRIBUTION OF ELECTRICITY 263 


to plastering only. In the case of concrete buildings the outlet 
boxes for lamps, switches, plug cut-outs, etc., must be located and 
firmly attached to the forms with complete conduit interconnection 
before the concrete is poured. The entire conduit system should 
form a complete metallic system which should be grounded. In 
this condition the installation provides practically absolute safety 
from mechanical injury, and when supplemented by proper cut- 
outs and fuse apparatus, from fires originating in short-cireuits or 
grounded wires. The only objection to this type of installation 
which has arisen is the condensation of moisture inside of the 
conduits. This has been known to take place to such an extent 
as to result in the rotting of the insulation of the wires due to 
their permanent immersion in water. This objection may be 
largely obviated by running the conduit so that there are no 
pockets in the system, and so that they have a pitch or slope towards 
some outlet. It is customary to run three-wire mains, feeders and 
duplex branches in one pipe. It is not permitted, however, to run 
more than one set in a single pipe. Reliance, therefore, is placed 
entirely on the insulating covering of the wires without space sepa- 
ration, and on the suppression of any arc or spark between con- 
ductors or between conductors and ground by the walls of the 
conduit. ‘'T'wo-wire service is now limited to the smallest instal- 
lations, the maximum number of outlets permitted by supply com- 
panies on two-wire service varying somewhat, but generally not 
exceeding 25. It is permissible to run the wires of either two- or 
_ three-wire service in a single pipe. The magnetic influence of the 
iron-protective covering in the case of alternating-current circuits 
has never arisen as a prohibitive factor. The running of a single 
Wire carrying alternating current in an iron pipe is prohibited by 
the large increase of the impedance of the circuit and by the heat- 
ing of the conduit due to hysteresis and eddy currents. A series 
of tests have been made by the author to determine whether two- 
and three-wire circuits in an iron pipe could result in any appre- 
ciable increase of the impedance of the circuit. Two No. 6B. &S8._ 
wires were separated the maximum distance permitted by the inte- 
rior diameter of a 114-inch conduit, being rigidly held in position 
by strapping to opposite sides of a strip of wood. At 60 cycles, and 
for currents between 40 and 80 amperes, there was an average 
increase in the impedance of the circuit over the value when the 
circuit was in air and not surrounded by conduit of 214 per cent. 


264 ILLUMINATING ENGINEERING 


It is obvious, therefore, that in the moderate lengths usually met 
with in interior illumination, this introduces no disturbing factor. 

The lighting of interiors by arc lamps fed from series circuits . 
is to be avoided. As already mentioned, these circuits operate at 
high voltage, and special precautions must be taken in insulating 
any such circuit within a building. In most localities the intro- 
duction of such circuits into buildings is prohibited. 

Multiple-connected are lamps are frequently used for the ighting 
of stores, factories, sheds, etc., and they are supplied by low- 
voltage distributing mains. In such circumstances due considera- 
tion must be given to the regulation of these circuits if incan- 
descent lamps are also to be operated from them. The are lamp 
takes from 4 to 9 amperes, and when this is the only type of lamp 
on the circuit the carrying capacity is often the determining factor 
rather than any question of regulation. The National Electrical 
Code prescribes the maximum values of current which it is per- 
mitted to carry on various sizes of wire. Each lamp or series of 
lamps, in case several are operated in series, must be provided 
with a fused cut-out. The general description and rules covering 
incandescent wiring, as already described, apply also to multiple 
are circuits, but the underwriters’ requirements prescribe certain 
additional regulations, which are duly set forth in the publications 
mentioned above. 

(b) Control. It is obvious that the entire system of an interior 
installation should be under control. We may define “control” 
as the possibility of individual and separate operation of all lamps, 
and the prompt cutting out of any portion of the system which 
may develop trouble. Thus every lamp or group of lamps should 
be operated by an accessible switch, and every branch circuit should 
also be equipped with apparatus permitting its easy separation 
from the remainder of the system. Individual distributing centers 
or the feeders supplying them should be equipped with switches. 
In addition to these essentials for manual operation, the whole 
system must be protected by fuses or automatic circuit-interrupting 
devices. It is highly essential that the main distributing center, 
the service connections, and all subsidiary centers should be in 
well-illuminated and readily accessible locations. 

In the denser sections of a distributing system, the service wires 
will usually be brought in from underground. Connection to a 
residence is usually made from a manhole permitting access to 


GENERATION AND DISTRIBUTION OF ELECTRICITY 265 


the underground network. The manhole is an essential part of a 
system of underground ducts. The building connection is usually 
made from these manholes by small conduit connection, this con- 
duit being made either of fiber, treated wood, terra cotta or any of 
the many types offered by the market. These conduits are brought 
through the building line underground, and the service wires 
brought above the surface by a continuation of the conduit or in 
iron pipe. ‘These conduits should drain back to the manhole, that 
is, away from the house, and after the wires are drawn in the 
conduit opening should be stopped so as to prevent gases from 
flowing into the building. 

In the outlying districts where the distribution is overhead 
various methods are used for bringing the service wires inside 
buildings. In many instances this is done by putting suitable 
bushings through the walls near the roof of the house. The best 
practice, however, takes the service wires from the transformer into 
an iron pipe some distance above ground level, the pipe leading 
below ground into the basement as already described. This pipe 
- connection should be provided at the top with a rain-proof bushing, 
and is particularly desirable in localities where there is a possi- 
bility of future underground service. The report of the committee 
on overhead construction of the National Electric Light Associa- 
tion, 1910, describes in detail various methods of making service 
connections. 

Interior-lighting systems, whether supplied from isolated plants 
or from public-service companies, should be equipped with a main 
switch controlling the entire system. Also each feeder should be 
equipped with a switch. The next subdivision at the distributing 
centers should provide either a switch or enclosed fuse for each 
two-wire branch. The main switch of the system, and the indi- 
vidual feeder switches, should each be equipped with fuses or sup- 
plemented by some form of automatic circuit-interrupting device. 
It is sometimes desirable to have a switch at the distributing 
center, although this is not necessary if the feeder furnishing this . 
center is so equipped. Branch circuits must be equipped with 
fuses, but not necessarily with switches. The underwriters’ re- 
quirements limit the capacity of a single circuit from a distributing 
center to 660 watts. This figure was probably originally based on 
the demand of twelve 55-watt carbon lamps. And, in general, 
branch circuits in the past have been limited to 10 or 12 outlets. 

10 


266 ILLUMINATING HNGINEERING 


It is now possible to run many more outlets to a branch circuit 
by the use of low-power tungsten lamps. The branch circuits for 
incandescent lighting are usually protected by fuses of 10-ampere 
capacity. These fuses are either of Edison “screw-plug” or of 
“ cartridge ” type, with present tendency to a return to the former. 
As already stated, the feeders must be protected by fuses, and for 
this purpose the “cartridge” fuse is best. In many large instal- 
lations the feeders are protected by circuit breakers located on 
switchboards of more or less elaborate design. ‘The requirements 
of theaters lead to especially detailed switching and regulating de- 
vices. Fuses are manufactured up to 500- and 600-ampere ca- 
pacity, but circuit breakers are preferable above the former figure 
on account of the cost of the fuses and of the time required for 
their operation. Flexible cable must be used in all conduit instal- 
lations, and may be had to accommodate practically any current. 
In the larger installations feeders frequently have a cross-section 
of 500,000 circular mils, and in extreme instances are even of 
greater size. The subdivision in these cases of the total capacity — 
required is highly advisable on the score of convenience of instal- 
lation. The installation of conduit of diameter larger than 2 
inches will usually involve difficulties unless special provision is 
made in the design of the building. Two-inch conduit will accom- 
modate three No. 00 wires; 3- and 4-inch conduit has been used, 
but 2 inches marks the limit for convenient installation. The 
neutral wire is made of full size in all interior wiring, so that 
when for any reason one side of the circuit is interrupted the 
neutral will provide full carrying capacity for the return current. 

(c) Cost of Interior Wiring. Since the prices of labor and ma- 
terial differ in different localities and at different times, it is 
dificult to state even approximately what the cost of distributing 
systems for lighting should be. In large cities, however, these 
variations are not very wide, and it is possible to state the limits 
within which the cost, expressed in terms of the usual contractor’s 
price per outlet, should lie. The figures given below apply to 
interlor wiring of all classes, from the small residence up to the 
large hotel or office building. They cover the portion of the work 
from the main source of supply, assumed to be at the building 
line. In case the building is lighted from its own plant these 
figures will apply to the portion of the installation lying between 


GENERATION AND DISTRIBUTION OF ELECTRICITY 267 


the lamp and the plant switchboard. No lamps, fixtures or re- 
flectors are included in these prices: 

Exposed wiring, $1.50 to $1.60 per outlet. 

Wire in wooden moulding, $2.00 to $2.50 per outlet. 

Concealed knob and tube wiring, $2.50 to $3.00 per outlet, with 
$1.00 added per switch outlet. 

Wiring in iron conduit and in new buildings, $4.50 to $5.00 per 
outlet. | 

Wiring in iron conduits in concrete buildings, $5.00 to $6.00 
per outlet. 

In the above, switches and base-board plugs are considered as 
outlets when the iron box is included. If the switch and plate is 
also to be furnished, approximately $1.00 per outlet of this nature 
should be added. For the larger installations in modern buildings 
the price of $7.00 per outlet, including all wiring and feeders 
up to the lighting fixture, has been found to be a fairly close 
figure. 

For that portion of the wiring which may be necessary beyond. 
the building line, as, for instance, the service connection and 
transformers, in those regions where alternating service is sup- 
plied, it is hardly possible to state even approximate figures of 
what the prices will be. The cost of wire follows that of copper 
more or less closely, and transformers vary somewhat in price. 
Lighting transformers suitable for erection on poles and for 60- 
eycle operation may be had in any capacity between 6/10 kw. and 
50 kw. As adapted to 1100 or 2200 primary circuits, and trans- 
forming to 110 or 220 two- or three-wire secondary, their price 
varies from $27.00 per kilowatt for the 1-kw. size to between $7.00 
and $8.00 per kilowatt for sizes in the neighborhood of 40 kw. and 
50 kw. The prices are somewhat higher for higher primary volt- 
ages, and ‘transformers adapted to location in subways are from 
10 to 12 per cent more expensive than the usual out-of-door type. 
Transformers for 25 cycles cost from 40 to 50 per cent more than 
those for 60 cycles. 

(d) Fire and Insurance Control. The National Board of Fire 
Underwriters, and in most places municipal regulations, require 
strict supervision of the installation of electric wiring. It is usually 
required that the electrical contractor shall secure a permit for 
any new work or repairs to electric wiring in buildings. In many 
eases the fire underwriters are satisfied with the municipal super- 


268 ILLUMINATING ENGINEERING 


vision and make no independent demands of their own. This is 
especially the case where the city adopts the National Electrical 
Code for its own regulations. Presumably this permit for wiring 
is followed up by an inspection of the work after completion, by 
a city official. Too often, however, this inspection is of the most 
perfunctory character. The inspector will almost invariably be 
content with a visual inspection of the installation. From the 
nature of the troubles and imperfections that are likely to arise 
from a system of wiring, electrical tests are the only ones which 
can yield complete evidence as to the state and the character of 
the work. Insurance and city authorities therefore would do well 
to require a thorough testing of every installation before approval 
and acceptance. Since there is at present no municipal regulation 
which ensures tests of this nature, the designing engineer should 
be careful to incorporate in his specification clauses requiring the 
complete testing by the contractor. This method of accomplishing 
the testing should be easily available to the city, which in yielding 
a permit could stipulate that before acceptance proper tests should 
be made in the presence of the city official. 

It has been already mentioned that the entire system of metal 
conduit of an interior installation should be grounded. Grounding 
means connecting as definitely and permanently as possible to the 
earth, thus maintaining the grounded portion at the potential of 
the earth. The neutral of underground direct-current systems is 
almost invariably grounded. Interior-wiring systems should, in 
the writer’s opinion, be always grounded. Ground connections may 
be readily made by connecting between the grounding point of the 
circuit and the metal pipes of the city water supply. Such connec- 
tions should be soldered and of fairly large size of wire. To en- 
sure a ground independent of water or gas pipes an iron pipe may 
be driven 5 or 6 feet into solid soil, the damper the soi¥ the better, 
and the ground connection soldered to this pipe. The conditions 
will be improved by using several pipes and by removing the earth 
from around the top of the pipe to a depth and diameter of about 
1 foot each, and then filling this hole with salt. 

There has been a wide discussion as to the advisability of ground- 
ing alternating-current secondary circuits. ‘These circuits are 
usually three-wire, and the ground connection should obviously 
be taken from the neutral. The great advantage of grounding the 
neutral is in the fact that should. the primary voltage reach the 


GENERATION AND DISTRIBUTION OF ELECTRICITY 269 


secondary wiring by the failure of a transformer or by the crossing 
of the respective lines, the high-voltage circuit thus brought into 
connection with the low-voltage wiring would be grounded and 
thus prevent arcing and danger to life. In many instances, also, 
electrostatic charges may be induced in the secondary wiring by 
disturbances in the primary circuit. This may result in serious 
shock to persons handling the secondary circuits if these circuits 
are not grounded. . 

The supposed objection to grounding such circuits is that it 
places the potential of one side of the three-wire system between 
the bare contacts on lamps and other devices and the ground, thus 
offering the possibility that persons receive shocks. The National 
Electric Light Association recommends that the grounding of sec- 
ondary circuits be limited to those on which the voltage of one side 
does not exceed 150. This means that no shock of a higher value 
than that stated could be received by anyone touching an unin- 
sulated portion of the circuit. The reasons for not grounding cir- 
cuits of higher potential do not appear to be good. There can be 
no question that the grounding of the circuit offers great pro- 
tection from any trouble that may arise from the primary circuit. 
This is undoubtedly the most likely and the most serious source 
from which trouble may come. The danger of shock to persons is 
hardly greater when the system is grounded than when it is not, 
and in those systems in which the voltage is carried to values dan- 
gerous to life it would appear desirable to provide the safeguards 
in other ways, such as complete insulation of all live contacts, or 
by other methods usual in high-voltage circuits. 

(e) Specifications and Contracts. In preparing specifications 
and making contracts for an installation it is highly desirable that 
each should be as complete and explicit as it is possible to make 
them. The specifications should always be accompanied by draw- 
ings. Of the numerous clauses for the protection of the client 
which should be inserted, none perhaps is more important than 
that applying to the charges for alterations or extensions of the. 
work, as set down in the specification. In competitive bidding 
on work of this nature a contractor will often look to his charges 
for extras and alterations for the best part of his profit. The 
engineer should therefore endeavor to describe on the drawings or 
by explicit statement every outlet of installation. General clauses 
should be inserted which shall protect the client during the process 


270 ILLUMINATING ENGINEERING 


of the work from damage to persons and property, and relieve 
him from all responsibility until the installation is ready to be 
turned over complete. In large installations the contractor should 
be required to place insurance on completed portions of the work 
and to give bond for its completion within the date stipulated in 
the contract. The specifications should cover carefully the sizes 
of all mains, feeders and branches, together with the conduit in 
which they are placed. Full details should be given of all switches, 
distributing boards, panels, etc. The trade names of manufactured 
articles which will be accepted should also be given, and the gen- 
eral statement made that no material not approved by the Board 
of Fire Underwriters may be used. The drawings should show the 
accurate location of all outlets, service connections, distributing 
centers and the run of all feeders. It is highly desirable that the 
engineer and architect should have early consultation so that the 
latter may know what space will be required by the engineer. ‘Too 
often the architect’s plans are completed before the engineer sees 
them. The architect, as a general thing, has a very limited knowl- 
edge of the requirements of an electric-wiring installation, and it 
is usually assumed that the illuminating engineer requires no space 
at all for his circuits. This consultation is especially advisable for 
buildings of reinforced concrete where it is inadvisable to pass 
conduit through reinforced beams. 

The drawings should also indicate the type of fixture, lamp, 
reflector, mounting height, etc. ‘The National Electrical Con- 
tractors’ Association has published a set of symbols which are in 


general use for indicating the nature and location of distributing ~ 


centers and the various types of outlet, ete. A standard set of 
symbols of this nature applying to the different methods of mount- 
ing lighting units and describing their character would be very 
useful. : 

The wiring for lighting systems is often installed on what is 
known as the time and material basis. This means that the con- 
tractor charges the cost of material used and the hours of labor 
required to the owner, with a certain percentage added. ‘This is 
rarely, if ever, a satisfactory method to the owner. To ensure a 
reasonable charge it requires a constant inspection of material and 
labor time. It will usually be possible to secure competitive bids, 
and then require the contractor to give the owner the benefit of 
any saving under the contracted figure which results from keeping 


} 
i 
. 





GENERATION AND DISTRIBUTION OF ELECTRICITY 271 


a record on the time and material basis. In such a case the con- 
tractor furnishes the engineer with a statement of material and 
labor time at regular intervals. 

(f) Tests. Satisfactory performance in wiring installations de- 
pends primarily on regulation and on the nature of the material 
and workmanship. The regulation will depend largely on the 
sizes of conductor specified by the engineer, and a test of regulation 
will only check up the methods which have been employed in 
making joints and contacts. A full-load test, however, should be 
invariably applied to the system before its acceptance. Every 
switch should be operated and each lamp socket and base-board 
plug tested. Insulation tests are rarely applied to interior-wiring 
systems. It is advisable, however, to apply at least double the 
normal operating voltage to the completed system. A stipulation 
to this effect should be included in the specification. The con- 
tract should contain a clause requiring the contractor to carry out 
the tests in the presence of the engineer and the details of this 
test should be given. 


2. Hzxterior Illumination 


(a) The commonest form of outside-lighting circuit is that of 
the series incandescent or arc system. ‘These circuits are usually 
run overhead, except in the more densely populated portions of the 
city. No special comment, therefore, seems needed as to the instal- 
lation beyond the regulations set down by the National Electrical 
Code. These circuits are of moderate voltage (from 2000 to 8000), 
and may therefore be handled by a variety of approved grades of | 
manufactured wire, insulators, ete. Series circuits are controlled, 
as a whole, from a generating station or substation, the entire 
protective apparatus being installed there. Special precautions 
may be necessary in some places for the protection of low-voltage 
lighting circuits and of telephone and telegraph wires. This class 
of service is more satisfactory when run in underground conduits, 
and this is usually required by the authorities in the centers of. 
large cities. The cities usually own the conduit system and rent 
space to the supply companies. The single conductor of the are 
or incandescent circuit is insulated with rubber or paper and the 
whole covered with lead. The manholes of the duct system are 
usually from 400 to 600 feet apart, and individual lamps are fed 
through branch conduits between the manhole and the base of the 


272 ILLUMINATING ENGINEERING 


pole. The cables then rise inside the iron pole to the lamp. Since 
there is little or no difference in potential between the two sides 
of such a loop from a manhole to a lamp, a duplex conductor may 
safely be used for this portion of the circuit. The lamp itself, 
however, should be insulated from its support, since it may receive 
the full potential of the circuit. Grounds on this class of circuit 
are very dangerous. The lead sheathing of underground cables 
usually affords sufficient protection between mains of different 
classes of service; thus are circuits are frequently run in the same 
duct with the low-potential multiple-distribution mains. Instances 
have been known in which trouble has arisen by reason of this 
proximity, but a rental charge on the part of a city of 5 cents 
per duct foot per annum is usually sufficient to cause the supply 
company to put as many conductors as possible in one duct. Hx- 
cellent data as to the construction of conduits, their cost, etc., may 
be found in the Standard Hand-Book for Electrical Engineers. 


3. Cost of Operation 


There is probably no phase of the general problem of electric 
lighting which attracts more public discussion than that of its 
cost. Public-service corporations, particularly if they have a 
monopoly of the consuming market, are naturally the objects of 
public suspicion. This is especially true of companies selling elec- 
tricity for lighting, and the explanation is to be found in the great 
discrepancy always existing between the admitted cost of electrical 
energy at the station bus-bars and the price at which it is sold to 
the consumer. The latter figure is often ten or more times as 
great as the former, and consequently is often the object of unin- 
formed public clamor. The reasons for the difference will be better 
understood after a discussion of some of the factors entering into 
the actual cost of generating and delivering electric power. 

(a) Cost of Electric Power. The commonest basis of estimating 
the cost of electric power is the summation of all expenditure nec- 
essary to deliver the power at the station feeder bus-bars ready for 
distribution. This total cost divided by the total energy generated 
gives the unit cost, i. e., the cost per kilowatt hour. This apparently 
simple method, however, will rarely yield the same figure for two 
different months, or weeks, or even days in the year, for the total 
cost of electric power is not directly proportional to the amount 
generated. 


GENERATION AND DISTRIBUTION OF ELECTRICITY 273 


The total cost may be divided into two classes: (1) fixed charges 
and (2) operating expenses. In the item fied charges are in- 
cluded all expenditures necessary whether or not the plant gen- 
erates power. Thus in this class fall the items of interest, taxes, 
insurance, depreciation and obsolescence. They represent the ag- 
gregate cost of having an up-to-date power station ready to deliver 
power. By depreciation is meant the outlay necessary to keep all 
generating equipment in repair, and to replace efficient apparatus 
worn out in service. By obsolescence is meant the cost of pur- 
chasing apparatus and equipment to replace that which has been 
rendered obsolete and inefficient by improvements and increased 
knowledge of the art. Interest and taxes expressed in per cent of 
the cost of the plant will not vary with the type of plant; insur- 
ance is often eliminated entirely in modern plants of fire-proof 
construction; depreciation and obsolescence vary widely with the 
type and size of plant, being greatest for reciprocating steam plants 
and least for water-power plants. The aggregate of fixed charges, 
in per cent of the cost of the plant, varies from 9 to 17 per cent in 
modern plants of size required to furnish city lighting service. 
The lower figure is reached only in the best type of water-power 
plant, and the upper refers to reciprocating steam engines oper- 
ating under poor conditions. The cost of the power plant varies 
from $80 per kilowatt of installed capacity, in the case of steam 
turbines, to $100 or $125 for reciprocating steam engines, and to 
$200 or more for water-power plants. Large gas-engine plants 
cost about $135 per kilowatt of installed capacity. 

The second class of expense in the production of power is called 
the operating expense, and it includes all items, such as fuel, oil, 
attendance, etc., which are approximately proportional to the 
amount of power generated. The proportionality between total 
operating expenses and amount of power generated is not exact, 
since the efficiency of steam and electrical apparatus is not the 
same for all values of the load upon them. With proper sub- 
division of the total capacity into smaller units, however, it is - 
usually possible to operate with machines loaded to more than 50 
per cent of their rated capacity, and in such conditions the oper- 
ating expenses per kilowatt hour are approximately uniform at 
all times. Average values of operating expenses in large stations 
are .3 cent per kilowatt hour for gas-engine plants, .4 to .5 cent for 
steam-turbine, and .6 cent for reciprocating-engine plants. 


274 ILLUMINATING ENGINEERING 


It is obvious that since the fixed charges are constant and the 
operating expenses proportional to the amount of power generated, 
the cost per kilowatt hour will be least when the station is gen- 

erating its greatest output. The minimum possible cost would be 
' reached if the station could operate continuously at its maximum 
capacity. In such a case, at 12 per cent fixed charges, an up-to-date 
steam-turbine plant could generate power at the feeder terminals 
at approximately .5 cent per kilowatt hour. Unfortunately, how- 
ever, the maximum load on the usual central station lasts a very 
short time, the load curve having a sharp peak in the late afternoon 
and early evening hours. The value of the maximum power output 
shown by this peak determines the capacity required at the central 
station. Consequently, at periods of light load, as for instance, 
during the morning hours, fixed charges must be paid on more 
generating equipment than are required to handle the load. This 
variation of the load throughout the day, in its effect on the cost 
of power, is described in terms of a quantity known as the “load 
factor,” which is the ratio of the average daily, monthly or yearly 
load to the maximum loads occurring in the corresponding inter- 
vals. The daily load factor then is a quantity less than 1, and 
represents the proportion of the maximum daily power output 
which may be multiplied by 24 in order to arrive at the total num- 
ber of kilowatt hours generated through the day. It is therefore 
highly desirable to increase the average daily load, and so render 
the load factor as near to the value 1 as possible. The load factor 
corresponding to lighting service only is very low, and lighting 
companies make great efforts to develop a day load comprising 
motors of all kinds, and heating, cooking and other domestic ap- 
pliances. The daily load factor of a large central station, which 
supplements its lighting load in every way possible, is about .50; 
the yearly load factor about .30. At load factor .50 the average 
total cost of generation in a gas-engine plant is .65 cent, in a 
steam-turbine plant .7 cent, and in a steam-engine plant about 
.9 cent per kilowatt hour. ; | 

(b) Systems of Rates for Sale of Power. In the early days of 
electric lighting it was customary to charge a consumer simply in 
terms of the number of lamps installed without reference to the 
number of hours they were used. This method of charging, known 
as the flat-rate system, was obviously unfair to the economical user, 
and meters for reading the total number of kilowatt hours were 


Oe, a 


GENERATION AND DISTRIBUTION OF ELECTRICITY 275 


developed as a basis for charging. This method alone, however, 
is obviously not equitable, since it costs the supply company more 
to supply a consumer during the time of peak load than at other 
times. Consequently, consumers are often classified on some basis 
representing the times of the day during which they take their 
maximum power, and different rates apply to the several classes. 
Such a classification might separate, for instance, the services to 
residences, to stores or factories, and to day motors. A further 
refinement in the methods of charging is found in the so-called 
two-rate systems, which aim to charge a consumer a higher rate 
for the power he uses during his peak hours and a lower rate for 
the remainder. ‘This method evidently aims to charge each con- 
sumer his proportionate share of the fixed and operating charges, 
respectively. The obvious difficulty is that of ascertaining the 
maximum load of each consumer. For residence lighting it is 
usually assumed that some proportion of the total number of lamps 
connected will be burned together for a definite number of hours 
each day. This number of kilowatt hours will then be charged 
for at the higher rate, and all power in excess at some lower rate. 
Mazimum-demand meters, which indicate the highest value of 
power taken during any chosen interval, have also been used as a 
means of arriving at the value of a consumer’s peak. This, how- 
ever, constitutes a separate measuring instrument for each con- 
sumer, and on account of the expense involved the plan has not 
as yet been widely adopted. 

The actual price at which power for Cereicen is sold varies widely 
in different places. In the larger cities the primary rate is rarely 
less than 10 cents per kilowatt hour, which may be charged, for 
instance, for all power up to the amount consumed by one-half 
the connected load if burned for 30 hours. All power in excess of 
this during the month would then be charged for at a less rate, 
say 7 or 5 cents per kilowatt hour. One reading per month of a 
meter indicating kilowatt hours, therefore, serves to fix the amount 
of the consumer’s bill. . 

The wide discrepancy between the prices at which power is sold 
and the cost of its generation have led to frequent agitation by the 
public of the question of regulating the rates for the sale of power 
by law. This type of discussion arising as well in connection with 
other classes of public-service corporations has led to the forma- 
tion in many states of public-utilities commissions, which have 


276 ILLUMINATING ENGINEERING 


the power to investigate and regulate the conditions of manufac- 
, ture and sale of the respective public commodities. The figures 
of cost of generating power which have been given apply at the 
station bus-bars. The discrepancy alluded to above includes the 
cost to the supply company of distributing the power to the con- 
sumers, the cost of meters and their regular inspection, and the 
general office expenses. While the cost of distribution, which in- 
cludes the capital charges on all the distributing system, as well 
as its inspection and maintenance, duct rentals, etc., is usually a 
much larger figure than at first apparent, the several items men- 
tioned do not bring the actual cost of delivering the power to the 
consumer very near to the figure at which it is sold. The remaining 
difference is not all profit to the company, however, but is in part 
applied to paying the obligations of early lighting companies, 
bought up by the present one, and defunct through obsolescence or 
other cause. It is worth noting that a recent careful investigation 
by a public-utilities commission of the rates charged by a lighting 
company in a large city in the middle West resulted in a decision 
that 14 cents and 8 cents per kilowatt hour were equitable primary 
and secondary rates. 


BIBLIOGRAPHY 


F. Koester: Steam Electric Power Plants. 

Franklin and Esty: Elements of Electrical Engineering. 

C. W. Stone: Modern Lighting Systems. Proc. A. I. E. E., June, 1910. 
Sheldon and Hausmann: Dynamo Electric Machinery. 

C. P. Steinmetz: General Lectures on Electrical Engineering. 

H. G. Stott: Cost of Power. Trans. A. I. HE. E., XXVIII, p. 1479, 1909. 
H. B. Gear: Diversity Factor. Proc. A. I. E. E., Aug., 1910. 

Reports to National Electric Light Association. 

H. Foster: Electrical Engineers Pocket-Book. 

Standard Hand-Book for Electrical Engineers. 


a a eo 


VII (1) 
PRINCIPLES OF MANUFACTURE AND DISTRIBUTION 


OF GAS, WITH PARTICULAR REFERENCE 
TO LIGHTING 


By EK. G. CowpEry 


CONTENTS 


Manufacturing. 
General characteristics of coal gas and water gas. 
Effect of different constituents on the calorific value and illuminat- 
‘Ing power of coal gas. 
Illuminants, their characteristics. 
Manufacture of coal gas. 
Open furnace heating of benches. 
Regenerative furnace heating of benches. 
Development in the retort under varying heats and conditions. 
Brief references to through retorts. 
Brief reference to vertical retorts. 
Brief reference to inclined retorts. 
Brief reference to by-product coke oven process. 
Purification of coal gas. 
Tar extraction. 
Cooling. 
Ammonia extraction. 
Sulphur extraction. 
Carburetted water gas. 
General statements. 
As made from fixed carbon, steam and oil. 
Development. 
Harris process. 
Tessie du Motay. 
Lowe process. 
Treatment of different oils. 
Paraffin base oil. 
Semi-paraffin base oil. 
Asphalt base oil. 
Basic claim Lowe patent. 
Efficiency of Lowe apparatus. 
Purification of water gas. 
Carburetted water gas made from oil and steam only. 
Producer gas. 
Metering gas at the manufacturing station. 
Gas holders. 


278 ILLUMINATING HNGINEERING 


Distribution. 
Low pressure. 
District holders. 
Reinforcing pressure mains. 
High pressure for suburban or long distance distribution. 
Semi-high pressure or “‘ Booster ” system. 
Formula for flow of gas through pipes. 
Low pressure. 
High pressure. 
Excessively high pressure. 
Location of gas works. 
Station governors. 
Design of a distribution system. 
Drainage of mains. 
Pipe joints. 
Brief mention: services, gas meters, house piping and photometry. 
Calorimetry. 


There are three characteristic ways in which manufactured gas 
is used, each of which, in its own sphere, results in its extensive 
employment as an agent for the production of artificial light. When 
burned without previous mixture with air, it produces a flame of 
considerable intrinsic brilliancy ; when burned after previous mix- 
ture with air, it produces a non-luminous flame of high tempera- 
ture; and, thirdly, the application of its explosive action, when 
mixed with air and ignited in the cylinders of gas engines, places 
certain grades of artificial gas among the most economical agents 
for the production of power. 

I shall devote myself mainly to a description of the principles 
involved in the manufacture and distribution of the various gases 
delivered by the artificial-gas companies of the United States, but 
attention is purposely called to the use of producer gas for the 
production of power, as being an important modern means towards 
conservation of energy, and this phase of the subject will be briefly 
presented. 

Kinds of Gases. ‘The gases we will eonlatdee are generally classi- 
fied as follows: 

Illuminating gas is divided into two great classes, coal gas and 
carburetted water gas. 

Coal gas in turn is divided into two sub-classes, viz., that pro- 
duced by the distillation of gas coal in comparatively small re- 
torts, and that produced by the distillation of coking coal in larger 
ovens. 


MANUFACTURE AND DISTRIBUTION OF GAS 279 


Carburetted water gas, on the other hand, is divided into that 
made from fixed carbon, steam and oil, and that made from oil and 
steam only. 

Producer gas is made by the action of steam or air, or both, 
upon fixed carbon. 

General Considerations. [rom this classification it becomes evi- 
dent that coal gas is produced analytically, distilled from certain 
kinds of coal, while water gas and producer gas are synthetically 
made, that is, built up from the action of several constituents upon 
each other in a manner to be described later. 

The results in each case do not widely differ, as is illustrated 
in the table shown as Slide 1. In this connection it is to be under- 
stood that the analyses shown are representative only, and not abso- 
lute, under all conditions. 

Producer gas has been omitted from this nls but will be con- 
sidered later on. 

It is to be noted that the use of water gas made from steam and 
oil, owing to local conditions of supply of the raw materials, is at 
the present time practically confined to the Pacific slope, where 
it is extensively used. It is particularly interesting to note how 
closely this gas compares in composition with coal gas, although 
produced from very different materials. 

In this country, generally, and in Europe and Great Britain, 
carburetted water gas is understood to be the gas produced from 
coal or coke, steam and hydrocarbon oils, as shown in the third 
column of the table. 


TABLE OF GENERAL CHARACTERISTICS OF COAL AND WATER GAS 


bt + 


Coal mae made yah Pic ba ae tak = g te 

. : . 2 > Og 

His a ES hoods 50 Bb vokes SE 

° © ons “tnd 3 mM HH 

Per cent by volume. 

Tiltminant?’ i... . 4.75 4.8 12.8 VUE 32236" 2 4.4 0 
PNA e Sas ca dv wes 2.8 1764.4 1.0868 38. 
JE SO eee u0-02 36.0 13.4 34.64 1009. 0.5529 5. 
PIVOPTOLOY os ite se es 47.04 49.7 38.9 $39.78 326.2 0.0692. 0 
Carbon monoxide .. 8.04 4.1 30.9 U.21". s2odn. OS6EE ea 
Carbon dioxide .... 1.60 13 2.8 2.62 0 EBLgS. eG 
OC GGEL ik aero ea 0.39 0.7 0.6 0.16 0 1.1052 0 
Bitropen iiiilemsds is 2.16 3.4 2.8 6.58 0 0.9701 0 





ROLE bets ivr dorsi. 3 100.00 100.0 100.0 100.00 


280 ILLUMINATING ENGINEERING 


Coal gas made Water gas made 
in rom 
Retorté. “vans. anciel amen aaa 
Specifici:sravity cn wee ees 426 ‘Dee .683 482 
BU ee Pe ee are one 678. 675. 682. 680. 
Candlepower: oo eeene Geo eG 15.8 22. - 19.69 
Cu. ft. air req’d for combustion 
one .cubici{oolst eet oe tan 5.65 5.63 5.74 5.81 


Note.—The B. t. u. per cubic foot of “ illuminants ” varies consider- 
ably in different gases. In computing the amount of air required for 
combustion the illuminants were assumed to have a composite formula 
of C,H,. 


In discussing this table it is to be noted that each gas differs 
from the other only in the relative proportion of the same con- 
stituents. ‘To indicate this more clearly, it is seen that coal gas 
contains less illuminants, usually more hydrogen, considerably less 
carbon monoxide, and less carbon dioxide than water gas. 

These characteristic features exercise a considerable effect upon 
the candle-power, calorific value and specific gravity of the gases. 
For instance, a smaller amount of illuminants means lower candle- 
power, usually lower heat value and higher specific gravity. More 
methane means lower candle-power, usually higher calorific value, 
but it is of lesser specific gravity than the illuminants. 

An increased quantity of hydrogen means lower candle-power, 
lower heating value and very much lower specific gravity. 

Carbon monoxide burns with a blue flame, and in itself has only 
a relatively low calorific value. Carbon dioxide, being the product 
of the combustion of carbon, when present in gas, decreases the 
candle-power and heating value, but increases the specific gravity. 

These comparisons are general only, and give the result of the 
effect of any one of these constituent gases, considered from the 
point of view of such gas only, without consideration of the effect 
of other constituents at the same time. For instance, it might 
conceivably happen that an increase in the proportion of methane 
would be accompanied by such a large decrease in the percentage 
of carbon dioxide and nitrogen that the candle-power might actu- 
ally be raised. In other words, it is necessary to look at the com- 
position of a gas as a whole, in order to arrive at a satisfactory idea 
of the various enumerated properties. 

Dr. William B. Davidson, of Birmingham, England, in his re- 
cent paper entitled, “ Experiments in Carbonization on the Bir- 


MANUFACTURE AND DISTRIBUTION OF GAS 281 


mingham Coal-Test Plant,” read before the British Institution of 
Gas Engineers in 1910, gives some interesting data on the effect 
of these various constituents on coal gas. An extract from the 
same appears as follows: 

“In this connection it is interesting to consider the effect of 
each of the main constituents of coal gas on both the illuminating 
power and the calorific value. On this subject, the information 
available in technical literature is both incomplete and incorrect, 
and I have therefore undertaken a series of laboratory experiments 
with the object of ascertaining the effect on candle-power of ad- 
mixtures of small quantities of different gaseous constituents. 

The effect on calorific value is already known. The approximate 
results are given in the following table, and apply alike to No. 2 
and No. 1 argand burners used with full flame. 


EFFECT OF DIFFERENT CONSTITUENTS ON THE CALORIFIC VALUE AND ILLUMI- 
NATING PoweErR oF CoAL GAS ON A BASIS OF 540 B. T. VU. 
AND 16 CANDLES. 


Calorific Illuminating ; 
Constituents. Value ower Ratio. 
Per Cent. Per Cent. 
OR 8 ) AARe ee Ee — 1.0 — 3.5 1 to 3.5 decrease 
OPM ee SH. — 1.0 — 3.0 1 to 3.0 e. 
Ni vis shleaty Stwriotoaba 3 5.1648 — 1.0. — 2.6 1 to 2.6 > 
PCT OMM er ti. — 1.0 — 2.7 MPA Kn Poo? 9 we 
OED Age hein Arg ae — 0.4 — 0.5 Latoc1.0 7 
18 Ry gs ae, ORG ee — 0.4 — 0.5 tO 120) 
Increase in calorific value 
SPUD GATE: Sala sett + 0.9 — 0.6 = twice the decrease in 
illuminating power. 
eS EG) SP ear a + 1.9 + 10.9 1 to 6.0 increase 
ENS NS ee Barer aera + 6.0 + 18.0 L to. 3,0 “5 
Celitcads Fite ike ot kh + 10.5 + 125 1 to 12 


Nore.—Gas saturated with naphthalene vapor at 60° F. contains only 
0.0085 per cent by volume of this constituent. The increase in candle- 
power, due to this small amount, is only 0.16 or 1 per cent. It should 
be understood the per cent of illuminating power given is theoretical 
and true only within narrow limits. ‘ 


The figures for carbon dioxide, oxygen and nitrogen have been 
confirmed by experiments with the large test plant. It calls for 
remark, however, that in short trials the effect of the admission 
of air was not nearly so drastic as was indicated by laboratory 
tests. This was doubtless due mainly to the fact that the iron 
oxide underwent a large rise in temperature and threw off certain 


282 ILLUMINATING ENGINEERING 


hydrocarbons—chiefly benzene—with which the water in the mate- 
rial had become saturated. In one instance, the admission of 3 
per cent of air appeared to effect no reduction at all on the multiple. 
In experimenting with air it is, therefore, necessary to allow the 
plant to attain equilibrium before starting the test, and to prolong 
the trial. 

It will be observed that the effect of an admixture of 1 per cent | 
of nitrogen reduces the candle-power by about 2.6 per cent. As 
it is this ingredient that varies most of all in the composition of 
coal gas as manufactured in this country, and seeing that the effects 
of carbon dioxide, oxygen, carbon monoxide and benzene have all 
nearly the same ratio, it follows from theoretical considerations 
that 2 per cent reduction of illuminating power for 1 per cent 
reduction of calorific value the result previously indicated is ap- 
proximately what we should expect to find.” 

For purposes of gas-engine use a gas should be able to withstand 
a relatively high compression without undue loss or premature 
explosion. Methane withstands high compression without change. 

However, it may be stated that in general, for illuminating gas, 
the illuminants, ethane, methane, hydrogen and carbon monoxide 
are all desirable constituents, because they all add candle-power 
or heating value, but carbon dioxide, oxygen and nitrogen are un- 
desirable because of the lack of these properties. 

Illuminants. The illuminants play a large part in the charac- 
teristics of candle-power and calorific value of both coal and water 
gas. Some of the more important of these compounds, with their 
special characteristics, are given in the following table: 


TABLE OF ILLUMINANTS 
Spec. Grav. Dlum. B.T.U, Cu. Ft. Air 


; Req. for 
Sertes Name onan, Cason ss 5 
CrH», |Ethylene | C.H, 0.9676 68.5 1588.0 14.355 

1 Propylene Cable 1.4514 aes 2347.2 21.533 

. Butylene C,H, 1.9353 123. 3099.2 28.710 

“ .  Amylene CsH4 2.4191 igs 3847.2 35.888 
C,H, Acetylene C.H, 0.8984 240. 1476.7 11.963 
CyrHen_-. Allylene C,H, 1.3823 eee 2227.1 19.140 

if Crotonylene C,H, 1.8661 eae 2975.6 26.318 
C,Hon-, Benzene C,H, 2.6953 349. 3807.5 35.888 

. Toluene C;H,; 3.1792 mane 4552.0 43.065 

re Xylene C,H 3.6630 ah, 5294.2 50.2438 

i Mesitylene C,H, 4.1468 stile 6108.0 57.420 


CyHon+2 Naphthalene C,,H; 4.4230 980. 5906.8 57.420 


283 


MANUFACTURE AND DISTRIBUTION OF GAS 


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284 ILLUMINATING ENGINEERING 


NoTe.—All volumes of gases and vapors are given at 60° F. and 30” 
pressure. Benzene, being a liquid under ordinary conditions, was tested 
for candle-power by mixing its vapor with hydrogen, and a slit burner 
used. 

Naphthalene, being ordinarily a solid, was similarly mixed with coal 
gas. 


—Coal-Gas Manufacture—As Produced in Retorts 


The art of coal-gas manufacture is over a century old. William 
Murdoch, in England, between the years 1792 and 1798, was en- 
gaged in experimenting with different coals, and in devising appa- 
ratus for their distillation. In 1797-1798 lighting by coal gas 
was actually accomplished, for Murdoch, by means of his experi- 
mental plant, first lighted up his dwelling house, and a short time 
later a much larger building at Birmingham. 

From these first attempts, coal-gas manufacture has been de- 
veloped to the present state of the art. 





Fic. 2.—Simple Retort Setting. 


Principles of Coal-Gas Manufacture 


The generation of coal gas from gas coal is a process of destruc- 
tive distillation. The solid coal is charged into the retort, which 
in laboratory parlance would be called a muffle, and the retort is 
heated externally. 

Figure 2 shows a setting, which, though too primitive for 
modern use, exemplifies the primary principles. Jt consists of 
a retort set upon parallel fire-brick piers having openings through 
them for the passage of the heated products from the furnace, 
a furnace for heating, an open space around the retort to per- 


MANUFACTURE AND DISTRIBUTION OF GAS 285 


mit its envelopment by the heated products of the fire, and a 
flue for the escape of the products. The retort of burnt fire-clay, 
3 inches thick, cross-section oval, D shape or circular, being open 
at the front end only, has bolted to that end a cast-iron extension 
called a mouthpiece, which, projecting from the front wall of the 
setting, is fitted with a gas-tight door, through which opening the 
coal is introduced and the coke withdrawn. At the top or side of 
the mouthpiece is an opening to which is connected a cast-iron pipe 
rising vertically, the upper end dipping into a seal of water. When ~ 
the charge of coal is placed into the heated retort distillation im- 
mediately begins, vapor and gases, air and steam being given off 
until the pressure is sufficient to overcome the seal in the dip-pipe, 
when the gas begins to bubble through and continues until car- 
bonization (by which is meant destructive distillation of the coal) 
ceases. The door is then opened for the withdrawal of the coke 
remaining in the retort and reintroduction of fresh coal. As soon 
as the door is opened there is a return of pressure in the retort 
to normal atmospheric, the water rises in the dip-pipe thereby 
preventing gas, from the collecting main from all the retorts, 
escaping through the open door. 

The practical extravagance of such a setting is at once apparent. 
Cold air enters through a shallow fire, burns to carbonic acid and 
steam, and the heated products pass around the retorts, and while 
still highly heated escape to the chimney. When the door is open 
for charging fresh fuel, which is usually hot coke withdrawn from 
the retorts, and when clinkering the fire, cold air sweeps over the 
fire directly around the retort, chilling it. Again, the combustion 
process is the one least suitable for surrounding, with combustible 
gases, retorts set some distance away, averaging 4 to 5 feet. Hav- 
ing but a short distance to travel through the fire, the conversion 
of the oxygen of the air into CO, is almost instantaneous, and the 
total heat of the chemical combination is confined to the fire, with 
the result that the fuel becomes heated to a temperature well above 
the fusing temperature of the ash. This rapidly seals off the fire, 
reducing the draft through it, and the combustion rate diminishes, 
cooling the setting, while the retorts are surrounded only by the 
products of combustion, and, except for the bottoms, immediately 
over the fire, get only the sensible heat of the products. Water 
is placed in the ash-pan so that a small quantity of steam rising 
therefrom may pass through the fire to assist in keeping down the 


286 ILLUMINATING ENGINEERING 


temperature of the fuel bed. This, while necessary to protect the 
grates, to a certain degree increases the difficulty, since the hydro- 
gen thus formed burns at once to water at the top of the fire, 
further localizing the intensity of combustion immediately above 
the surface of the fire. The result is, that uniform heating of the 
retorts is difficult and uneconomical. Thirty years ago this style © 
of setting was in wide-spread use. By having a large mass of 
fire-tile and small retorts, however, good results, as far as the 
quality of the gas was concerned, were obtainable. 

The difference between heating a setting of retorts and a boiler 
fire, for instance, is readily understood. In the latter case com- 
bustion must have progressed to near completion before the com- 
bustible products impinge on the comparatively cold tubes or shell 
and combustion is arrested. In a setting of retorts, where all 
parts are kept at a temperature well above the ignition point of 
the most dilute gaseous combustibles, it is desired that the fuel 
bed should be kept at a temperature just sufficient to carry on the 
chemical reaction for the conversion of the atmospheric oxygen 
into carbon monoxide, and the final combustion of that gas occurs 
around the retorts situated at a comparatively remote distance 
above it. Sahene 

The solution of these difficulties led to the adoption of the re- 
_ cuperative—sometimes called regenerative—method. Here there is 
a furnace below an arched chamber containing nine retorts exposed, 
except where supported, to the envelopment of heated products. 
This arched chamber and its contents of retorts is called a bench. 
Continuous arches so filled are called a stack of benches. The 
heated products of combustion on their way to the stack are led 
through passages made by thin fire-clay tiles; the primary air in 
its passage to the ash-pit, and the secondary air in its passage to 
the nostrils above the fire, pass around these tile flues, absorbing 
heat that was wasted in the former setting. Again, the fuel bed 
was deepened so that the oxygen on entering the fire, being first 
converted into CO,, passes up through more fuel and becomes re- 
duced to CO. We have now gaseous firing. There will be stored 
in the fuel bed only the heat developed by the combustion of car- 
bon to carbonic acid, and there will be abstracted from the fuel 
bed the heat absorbed by the separation of the hydrogen from 
oxygen of the steam, the reduction of the carbonic acid to carbon 
monoxide, and the increase in the sensible heat of the escaping 


MANUFACTURE AND DISTRIBUTION oF GAS 287 


atmospheric nitrogen. The top of the furnace is covered with 
a heavy covering of fire-tile built with an opening for the passage 
of the combustible CO diluted with N to the setting above. At 
this point, the highly heated secondary air combines with the gases 
from the fire and combustion at high temperature ensues. The 
fire, meanwhile, being deeper, has an arrangement by which false 
grate bars can be driven in at clinkering time, some distance above 









a, aay 
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1 z 
a ies 
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Bench of Nine Retorts, with Full Depth Recuperators. 
Fig. 3. 


the fixed grates, holding up the fire while the clinker is being 
removed from between the false and fixed bars. There is also an 
arrangement by which small streams of water drip through the 
bottom of the fire, reducing the temperature further. 

This method with care, gives satisfactory results, and is in 
extended use to-day. A further improvement in conditions is 
now obtained by returning a small quantity of the products of 
combustion through the fire, diluting the oxygen of the air and 
prolonging the period of combustion until all the retorts are bathed 
in flame. 


288 ILLUMINATING ENGINEERING 


Modern practice requires careful attention to bench heating. 
The per cent of combustible in the stack gases, the quantity of 
primary and secondary air are accurately read by meter and pro- 
portioned, and the heats of the combustion chamber and through- 
out the setting are noted at frequent intervals, with the result 
that uniform heats in the retorts at from 1700° F. to 1900° F. are 
maintained with an expenditure of fuel per ton of coal carbonized 
much less than formerly obtained. A retort, when discharged of 
its coke, should show a uniformly heated interior surface through- 
out—bright red in color. 

The angle at which the retorts are inclined to the horizon is a 
question of much importance. But before we have sufficient data 
to take up a discussion of this question, we must look into what 
goes on inside the retort. 

After a century of close study, it may be said, with regret, that 
not all of the details of the chemical reaction attending the con- 
version of coal into gas by the retort process are known at this time. 

The process is considered as occupying three stages: 

In the first one, a quantity (about 350 pounds) of cold, damp 
coal is charged into a retort 9 feet long and approximately 14 
inches by 26 inches in cross-section; then there is a rapid cooling 
taking place on the inner surface of the retort. There is an ab- 
sorption of heat by the coal, due to the high thermal head existent, 
and the heat rendered latent by the immediate evaporation of the 
water and the more volatile vapors in the coal. We know that the 
distillation begins on that portion of the coal in contact with the 
sides of the retort. | 

When coal is‘ heated in a closed vessel at a heat of about 660° 
to 700° F., fusion of the coal commences and hydrocarbon vapors 
begin to come off. If these vapors were condensed, they would be 
found to be mainly paraffins and olefins. 

The second stage ensues in which these hydrocarbons, meeting 
with the higher temperatures, begin to be affected. It is believed 
that there is a rearrangement and loosening of the C-H and the 
C-C bonds, and other compounds are formed. In the third stage 
the heat of the interior of the retort rises still higher, the reactions, 
almost instantaneous in many instances, are most complex, and so 
far have resisted entire elucidation. The aliphatic hydrocarbons, 
that is, the open-chain series, paraffins and olefins, as found in 
gas coal and petroleum, are, on the one hand, loosening their 


MANUFACTURE AND DISTRIBUTION oF GAS 289 


carbon bonds and splitting off the initial or simplest members of 
their series, while the residues unite into more complex closed- 
chain or aromatic compounds, such as benzene, toluene, xylene, 
ete. These benzol compounds, under the influence of heat, in time 
are decomposed with the liberation of hydrogen, carbon and the 
formation of still higher ring compounds. On the other hand, 
the free hydrogen present reacts on the aliphatic hydrocarbons. 
In the meanwhile, the oxygen and the nitrogen in the coal are 
forming other combinations, some of the nitrogen going into am- 
monia and some of the oxygen uniting to form phenols. 

A West Virginia coal would have a hydrocarbon component that 
is expressed as approximately C,,,H,,,O;o. 

This third stage is the one which does most to determine the 
candle-power and heating value of the gas obtained. The retort 
is filled to about 40 per cent of its volume with coal. After the 
water and first vapors are driven off, the coal continues to fuse 
and the evolution of gas becomes more rapid, and, passing above 
the coal, is exposed to the highly heated sides and top of the retort. 
The hydrocarbons and other vapors pass off in gradually decreas- 
ing proportion during the distillation period of the charge, which 
we are now considering as being of about 4 hours’ duration, and 
as the volume becomes less the energy expended on them dimin- 
ishes and the retort gradually increases in temperature toward the 
end of the period, at which time, the temperature being higher, 
the flow of gas slower, the effect ge Naas upon the gas is con- 
stantly changing. 

I have so far asked your attention to the consideration of that 
form of modern retort setting in which the charge of coal is dis- 
tilled in the shortest time, generally 4 hours. This design, being 
only a mechanical improvement upon the century-old chemical 
distillation of coal—an improvement looking toward economy in 
heating the bench and procuring more “even” heats, except so 
far as those bettered conditions could—did nothing to improve 
the chemical reactions. The distillation of coal is still conducted 
under very different conditions at the beginning than at the end of 
the period, and the gas emanating from the coal in the back of 
the retort is exposed to different heating conditions than that in 
the front. 

Avoiding a too technical and voluminous discussion of these 
changes, it will still be well for me to make a simple statement 


290 ILLUMINATING ENGINEERING 


of the most important changes occurring in this third stage of 
how vapors of the paraffin and olefin series, which are those coming 
from the second stage, are affected by the temperatures of the 
retort. 

It is doubly important to the gas engineer, because the same 
reactions occur in a water-gas apparatus or in an oil-gas plant, 
where paraffin base oils are subject to the “ cracking-up ” process. 
It constitutes one of the chief sources of interest and study of 
the gas engineer, and a better knowledge is sure to be rewarded 
by more economical operation of a gas plant, a better profit and 
improved product. 

The higher members of the paraffin series and olefin series break 
down even at temperatures below their boiling points, under normal 
pressure, to lower hydrocarbons of the same series, and the paraffins 
to some extent are converted into the olefin series. Under con- 
tinued exposure to these high temperatures, the lower paraffins and 
olefins are converted into members of the benzene series with de- 
posits of free carbon; if the heat still continues there is a produc- 
tion of acetylene, followed at once by a breaking down into marsh 
gas and a large deposit of free carbon. Benzene (C,H,), the lowest 
member of the benzene series, at ordinary temperatures exists as a 
vapor. It has a high illuminating value, and, in water gas made 
from some oils, contributes largely to the illuminating power, 
though not so much to its heating value. 

It is clear that the “ cracking-up ” process cannot go beyond the 
benzene-forming period without disastrous effect on the value of 
the gas, and it is true, further, that the formation of the benzenes 
are a loss in candle-power value over what would have occurred 
if the olefin gases, such as ethylene, had not been broken up. In 
other words, if we could convert the paraffins and olefins all into 
members of the olefin series, gaseous at ordinary temperatures, 
the highest efficiency would be realized, but in the rush of gas 
through the retort all the reactions are taking place at once. While 
some of the heavy paraffins and olefins are breaking down into 
lighter members of the same series others are being converted into 
benzols, while some of the benzols are going into hydrogen and 
free carbon. We must, therefore, use a heat which will crack up 
all the heavy paraffins and olefins, remove the gas before the final 
general breaking down occurs, and expect some losses in the process. 

The difficulties that the coal-gas engineer has to meet are now 


MANUFACTURE AND DISTRIBUTION OF GAS ag 


evident. He must maintain a heat in his retorts that will secure 
the proper “cracking up” of the heavier rush of gas in the first 
hour, and he must expect, in the form of retort under discussion, 
that there will be, toward the end of the carbonization period, too 
great an exposure to the heat of the smaller volume of gas, break- 
ing down into free carbon and methane—a non-illuminating gas. 

Other designs of retort settings suggest themselves as better than 
the one we have so far discussed. Instead of withdrawing the coke 
from the same door through which the coal was charged, retorts are 
used in many installations which open at both ends. The coal is 
charged into the retort until it is nearly full, and the coke is 










eal RODS LPI TE: (am 

UQAKRAANN SOONER NOVA RSE 
oes Liddbdabdddde aE og 
q 


Fig. 4.—Retorts. 


pushed out through the other end, the operation of pushing the 
coke out and recharging the retort being done by machinery in one 
motion. By this means the gas from the coal flows through the 
retort more rapidly; by reason of the smaller area existing between 
the coal and the top and sides of the retort, the temperature of the 
retort is reduced and a longer time is given for the carbonization 
period. There is still, however, direct exposure of the gas to the 
radiant heat from some portions of the retort. ; 

Another development is in the vertical retort. Here the coal 
is charged into the top and the coke taken from the bottom of a 
vertical retort, which usually tapers to somewhat larger at the bot- 
tom. Here the coal is fused, filling the retort; there is no appre- 
ciable amount of space between the coal and the sides of the retort 
for the gas to be highly heated, and the gases must flow, in a large 


292 ILLUMINATING ENGINEERING 


part at least, up through the central unfused core of the coal itself, 
thereby escaping the difficulty under discussion. That there is less 
breaking down into free carbon, marsh gas and hydrogen in the 
vertical-retort process than in the horizontal is apparent by the 
smaller percentage of free carbon extracted from the gas by the tar 
in the after processes. ‘The coal is raised to a greater altitude 
than in the horizontal retort, and when charged into the mouth 
at the top, which by machinery can be done with little labor, falls 
of its own weight out of the bottom as coke. Vertical retorts are 












) 


G Pa OSLLILELO LT DONO IIL ES // 


jae 


= = 
ZZ IIIT IIT III 














| we 


Fig. 5.—Vertical Retorts. 


in wide use in Europe, having superseded inclined retorts, which do 
not appear to suit the theoretical conditions as well as the verticals. 

The coke oven is another attempt at the solution of the problem 
of getting uniform, moderate heat throughout the body of the coal 
and throughout the carbonization period. It is, in effect, a large 
“ double-end ” horizontal retort in which large quantities (6 to 8, 
sometimes 10 tons) of coal are exposed to carefully graduated but 
moderate heats for from 20 to 36 hours. 

What effect upon the cracking up of hydrocarbons, of tempera- 
ture versus heat has, can hardly be discussed by me now. What is 
the relative effect of long-continued exposure to moderate tempera- 
ture, to quick exposure to high temperatures? Some of our leading 





MANUFACTURE AND DISTRIBUTION OF GAS 293 


developers of the chemistry of gas manufacture, notably the veteran 
Young, probably the most original, as he was the pioneer in this 
field, maintain that radiant heat has a very different effect on 
“ cracking ” than conducted or convected heat. 


CAST /ROW BOX 





Fig. 7.—Sketch of Purifier. 


Purification of Coal Gas 


The principal impurities in coal gas, which must be extracted 
before the gas is fit for commercial use, are tar, ammonia, sulphur 
and sometimes cyanogen. In connection with purification the sub- 
ject of condensation will be treated. 

The fundamental principle of condensation is to reduce the gas, 
during its passage through the works to a proper temperature, so 
that in its distribution through the gas mains to the consumers’ 


294 ILLUMINATING ENGINEERING 


appliances no vapors will condense out of it. In other words, after 
proper condensation at the works, the gas is, generally speaking, 
in a permanent fixed form for the ordinary conditions of dis- 
tribution. 

The principles employed in condensing coal gas are as follows: 

First, gradual reduction in temperature down to about 110° F. 
Above this point, as much of the tar as collects in the hydraulic 
main and foul mains is allowed to pass off into the tar well. If 
coal gas at or below 110° F. is allowed to remain in contact with 






je 





Fic. 8. 


coal-tar a great amount of the heavy hydrocarbons in the gas 
are absorbed by it. By draining the tar off at proper points in the 
process, the benzol and other heavy vapors are retained in the gas. 
Some tar is always carried with the gas through the various 
works pipes, and serves to absorb excess naphthalene vapors. 
After the primary condensation down to about 110° F., a further 
extraction of tar takes place. . This is accomplished in various ways, 
such as hot washing, or scrubbing, by centrifugal force, or mechan- 
ically, as in a P. & A. tar extractor, where the particles of tar are 
projected by high velocity against metal suntare? where they are 
deposited and run off. : 


MANUFACTURE AND DISTRIBUTION OF GAS 295 


The condensation principle of gradually cooling the gas is im- 
portant, as this prevents the sudden shocks to the gas, with at- 
tendant losses of valuable hydrocarbon vapors. Certain hydro- 
carbon vapors possess the property of apparently carrying other 
hydrocarbon vapors in a so-called state of suspension, up to the 
saturation point, which varies with the temperature. 

Naphthalene, Coal Gas. The subject of condensation would be 
incomplete without brief reference to naphthalene. Its formation 
is believed to be principally due to the latter-day high heats of 


an 


















Vertical Atmosphere Condanser: 

















» Vertical:Muleitubular Condenser Longitudinal Mugitubular Condenser 


Condensers 


iras.o: 


carbonization, and where it occurs in quantities it becomes ex- 
ceedingly troublesome. Recently, washing the gas with certain 
oils has proved very successful. In mixed coal- and water-gas- 
plants naphthalene is very readily handled, owing to the fact that 
the rich hydrocarbons in water gas absorb and carry it along. 

The mechanical principle employed in condensers is simply the 
transmission of the heat, either sensible or that freed by reason 
of the latent heat of condensation of vapors, through steel, usually 
tubes, to air or water which are used as the mediums for absorption. 


296 ILLUMINATING ENGINEERING 


Ammonia is extracted from coal gas by the well-known principle 
of the power of water to absorb it. The mechanical methods of 
doing this are by so-called washing and scrubbing. In the earlier 
stages of the process it is advisable to wash or scrub the gas with 
crude ammoniacal liquor, which assists in removing tar, CO,, H.S 
and CS, from the gas. The crude liquor also extracts ammonia. 
Of course, the fina] traces of ammonia are eliminated by the use 
of fresh water. 

Sulphur exists in crude gas as H,S, and also organic compounds, 
the latter being largely CS,. Washing or scrubbing the gas with 





AS MoniachE LIQUOR 


Fie. 10.—Water-Cooked Condenser. 


crude ammoniacal liquor extracts a portion of these compounds, 
which form various chemical combinations with NH,. A recent 
system of treating the gas, called the Feld system, eliminates usu- 
ally by far the greater portion of H,S, also some organic sulphur 
is removed in the purifiers. 

In the United States iron oxide is used in the usual system of pur- 
ification. The H,S in the gas combines with the iron oxide to form 
iron sulphide. The “fouled” material, by exposure to air, revivi- 
fies, the oxygen of the air combining with the iron sulphide to form 
iron oxide, leaving the sulphur in the material in the free state. 


MANUFACTURE AND DISTRIBUTION OF GAS 297 


The free sulphur probably does extract a certain amount of CS, 
from the gas, as CS, dissolves sulphur. 

In England the hydrated form of quicklime is employed. ‘This 
process removes CS, as well as H,S, but is not much used in this 
country on account of the expense. 


Carburetted Water Gas as Made from Fixed Carbon, Steam and Ou 


It is not the intention to present for your consideration mere 
history, but a brief reference to the development of carburetted 
water gas may not be out of place, and will probably assist in the 
clearer understanding of the principles underlying this process. 

















vy e@ ) \J ) Y \. at 
A NN [\ \\ ch 5 


A-S- 3-93. 


Fie. 11.—Plate No. 2—Rotary Scrubber. | 







The fundamental chemical principles underlying the process of 
making this gas from fixed carbon, steam and oil are compara- 
tively simple. In the first place, there is a bed of fuel, brought up to 
high temperature, which we may call incandescent carbon for the 
purposes of this lecture. Steam is admitted and passed through 
this fuel, and, as is well known, decomposes into its elements hydro- 
gen and oxygen in the presence of incandescent carbon. To give 
you an idea of such reactions, and the approximate minimum tem- 
peratures at which such decomposition takes place, whether in the 
presence of incandescent carbon or not, the following table is 
shown: 

~H,O<>H+0. Min. temp. about 1000° Cent.—1200° Cent. 

H,O+C=CO+H. Min. temp. about 600° Cent. | 
From this you will note the comparatively low temperature re- 
quired to decompose H,O in the presence of incandescent carbon. 


298 ILLUMINATING ENGINEERING 


The result of this reaction, which takes place in a fire-brick-lined 
vessel called a generator, is the formation of so-called blue or 
uncarburetted water gas, which consists principally of carbon mon- 
oxide and hydrogen, and burns with a blue practically non-lumi- 
nous flame, and has a calorific value of about 320 B.t. u. per cubic 
foot. | 

This blue gas then passes into a fire-brick-lined vessel filled with 
a checker-work of fire-brick, which has been heated to incandes- 
cence. A spray of hydrocarbon oil is admitted above this checker- 
brick, is vaporized and gasified by the heat, and mixes with the 
blue gas previously described. The oil furnishes the illuminants 
necessary for candle-power, and from the analysis submitted in the 
early part of this lecture, it will be seen that a good calorific value 
is also obtained. The candle-power and calorific value depend very 
largely upon the relative quantities of blue gas and the gas re- 
sulting from the decomposition of the oil. 

The mixture of blue and oil gas is subsequently subjected to a 
so-called “ fixing” process, by being passed through an additional 
amount of heated checker-brick, the effect of which is merely to 
render the various hydrocarbon gases more permanent under ordi- 
nary temperatures, probably by the reason of the decomposition or 
partial decomposition of some of the richer hydrocarbons into the 
simpler and more stable forms. 


Development of Water Gas 


The production of water gas has been attempted in three ways: 

First. In the earlier forms it was attempted to produce water 
gas by contact of steam with heated coal or coke contained in a 
retort externally heated, as is illustrated by the Harris patent. 


DESCRIPTION OF THE HARRIS PATENT 

A bench of three clay retorts, shown in Figure 1, was used. Re- 
tort A, or the decomposing retort, was provided with a perforated 
tile (Fig. 3). The retort was filled above the tile with anthracite 
coal broken to the size of an egg. 

Retorts B and C were filled with rich cannel coal. Figure 2 
shows a cross-section of the decomposing retort. Figure 4 il- 
lustrates the steam drier which was placed near the base of the 
furnace. Figure 5 represents the steam superheater. | 

Steam supplied from a boiler heated by the waste gases from 
the bench was first passed through superheater E into retort A, 


299 


MANUFACTURE AND DISTRIBUTION OF GaAs 


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300 ILLUMINATING ENGINEERING 


passing through the distributing tile D into the highly heated 
anthracite coal. Leaving this retort the gas was conducted to the 
rear end of either of the lower retorts, B and C, through pipes H, 
and from this retort through stand-pipes K to the hydraulic main. 

The gas from the decomposing retort was supplied to one bitumi- 
nous retort until the rich hydrocarbon vapors of the charge in 
this retort were exhausted. | 

This retort was then closed off by means of cock 3 and the gases 
from retort A were then passed to the other, ete. 

Retorts B and C were charged at intervals of about 2 hours. 
These attempts were unsuccessful, but your attention is directed 
to them to illustrate the basic principles. Various patent appli- 
cations, from time to time, show the recurrence of this idea in 
different men’s minds. The reason of the failure of this process 
is because the chemical reaction of steam upon the fixed carbon 
of the incandescent coal or coke is an endothermic one, in other 
words, one which absorbs energy in the form of heat, and requires 
much more heat to maintain it, and more intimate association of 
the steam and coai or coke than can be obtained in this way. 

Second. 'The next step in the process is embodied in the ideas 
formulated by Tessie du Motay. In general, this process consists | 
in making blue-water gas intermittently in a generator and storing 
same in a_ holder. 

The apparatus consists of generator A, gas-relief holder, bench of 
retorts with furnace C, retorts D, hydraulic main E, and naphtha 
vaporizer B. 

The generator is filled with anthracite coal or coke, through 
which steam is passed, after this bed of fuel has been brought to 
incandescence; the resulting gas being a blue-water gas, largely 
CO and hydrogen, this gas being passed along to the relief holder 
for storage. ‘The bench of retorts having been brought to the 
proper heat for vaporizing, the oil gas is admitted to the front end 
of retorts at point “ H,” and at the same point naphtha vapor is 
admitted, the naphtha having been vaporized in vaporizer “ B” by 
means of steam coils or otherwise; the naphtha vapor and blue- 
water gas are each regulated at this point, “ H,” to produce the 
proper candle-power of gas, and passing through the retorts “ D,” 
coming in contact with the heated surface is sufficiently heated 
to be largely converted into a fixed gas, passing off at the opposite 
end of the retorts to the hydraulic main, afterwards treated in a 


301 


MANUFACTURE AND DISTRIBUTION OF GAS 


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302 ILLUMINATING ENGINEERING 


similar manner to other gases. In operating, the generator was 
first brought up to heat by blowing sufficient air through a bed of 
fuel to raise this bed of fuel to a high temperature. When the 
fuel was hot enough, the blast was cut off, a valve closed and steam 
admitted, which, on passing through the fuel, resulted in the pro- 
duction of blue-water gas. The endothermic action of decomposi- 
tion of steam in the fuel bed resulted in a rapid cooling of the 
fire. When the fire temperature became so low that the steam was 
no longer readily decomposed, the admission of the steam was dis- 
continued, and the blast turned on again, as before, and the cycle 
of operations repeated. 

In the meantime hydrocarbon oils were being vaporized in a sepa- 
rate apparatus, and these vapors, mixed with the blue-water gas, 
were passed through an apparatus externally heated, wherein the 
gas was “ fixed” or rendered permanent. 

The limitations of this system of gas manufacture were that 
the oils which could be vaporized were the refined fractions of 
crude oil, called naphtha, and as these oils rapidly advanced in 
price the limit of economical operation on a commercial scale was 
soon passed. 


The Lowe Process 


Third. ‘The Lowe process. This method, or modification of it, 
is the one in use to-day. ‘To describe its essential principles it is 
advisable to insert a short description of the apparatus used. A 
Lowe water-gas set, or its equivalent, consists of— 

First. A generator, or vessel built of an iron shell with a fire- 
brick lining, and containing a deep bed of fuel. 

Second. A carbureter, or vessel consisting of an iron shell lined 
with fire-brick, and filled with a checker-work of fire-brick. This 
vessel has an open chamber at the top into which the oil is sprayed. 

Third. A superheater, or vessel built and checkered similar to 
a carbureter. 

To explain the operation of such a set we will first assume it 
cold, but with a coke or anthracite fire started in the generator. 
By means of a blower an air blast is turned under this fire, and the 
carbon in the fuel bed burns partly to CO,, partly to CO. The CO,, 
on passing through the incandescent fuel bed, is practically wholly 
_ decomposed to CO, the amount depending on blast velocity, tem- 
perature, etc. 


MANUFACTURE AND DISTRIBUTION oF GAS 303 


When the producer gas (for such it is) reaches the top of the 
generator above the fire it consists principally of N, CO and a small 
percentage of CO,. By means of a large fire-brick-lined connection 
this producer gas is conducted to the top of the carbureter. Here 
an additional blast opening introduces fresh air, and a portion or 
all of the CO in the producer gas burns to CO, in the carbureter. 
The resulting mixture passes out of the carbureter, and into the 
bottom of the superheater, where still another blast admits enough 
air to burn the remaining CO to CO,, in case it is desired to heat 
the superheater higher, but if not, no further air is admitted here. 
The final waste gases then pass out of the stack valve at the top 
of the superheater and escape into the atmosphere, or are first 
passed through some apparatus to abstract as much of the remain- 
ing sensible heat as possible. This process of blasting or blowing 
is continued until the entire fuel bed is highly incandescent, the 
checker-work in the carbureter at a high heat, and at a reduced 
temperature in the superheater. ‘T’he set is then ready to make gas. 

The blast is first shut off from all of the vessels, and the stack 
valve on the superheater closed, live steam is then turned into the 
generator below the fire. The resulting reactions are very instruc- 
tive. The H,O vapor is first decomposed by the incandescent car- 
bon to hydrogen and oxygen. This reaction is endothermic, that is, . 
heat is absorbed in doing this work. The hydrogen passes through 
the fire unchanged. 

The oxygen, on the other hand, immediately combines with car- 
bon to form CO and CO,, and every pound of carbon thus burning 
to CO, gives off about 14,544 B. t. u., the reaction being exothermic. 
The CO passes on through the fire, but the CO,, in the presence of 
the incandescent carbon, decomposes to CO, the reaction being 
endothermic. 

The gas appearing on the top of the fire, then, is a mixture of 
hydrogen and carbon monoxide, in practically equal proportions, 
together with a small percentage of CO, and some impurities. 
This mixture is the so-called blue or uncarburetted water gas, and © 
is merely one form of producer gas, having a calorific value of 
about 320 B. t. u. 

It will be noticed that the reactions in the generator are mostly 
endothermic, and, in fact, the fire is cooled very rapidly during the 
admission of steam, a run being generally from 5 to 10 minutes, 
at the end of which it is necessary to blast again. 


304 ILLUMINATING ENGINEERING 


Coming back to the blue-water gas, so-called because it burns 
with a blue flame in air, we find upon leaving the top of the gen- 
erator that it passes into the top of the carbureter. Here it meets 
with a spray of oil. This is sometimes the crude oil, but more 
often a gas distillate, which is the fraction obtained from crude oil 
after distilling off the gasolines and kerosenes, and stopping before 
the heavier lubricating oils appear. 

This oil, coming into the top chamber of the carbureter, vapor- 
izes under the intense heat and, mixing with the blue gas, starts 
through the carbureter. The lower portion of the carbureter and 
the superheater are merely heated checker-work for rendering the 
gases permanent under ordinary conditions, or “ fixing” it, as it 
is called in operative parlance. 

Crude petroleum consists of a mixture of a great number of 
definite hydrocarbons, that is, hydrocarbons that may be designated 
by exact chemical formulae, but which are so almost inextricably 
mixed in the oil that the separation of any one of the hydrocarbons 
in considerable quantities requires repeated distillations under fa- 
vorable conditions and chemical treatment. | 

Crude oils are designated as paraffin base, semi-paraffin base and 
asphalt base, according to the general character and composition 
of the oil. 

Paraffin-base oil, as I have stated in discussing coal-gas manu- 
facture, is one made up almost entirely of members of the paraffin 
and olefin series. Paraffins from simple CH, methane to penta- 
tricontane C,,H,, have been isolated; methane CH,, the simplest 
member existing as a gas; pentatricontane (C,,H,.), as a solid, 
melting at 76° F. This oil is found in the northern oil districts, 
such as Pennsylvania and Ohio. 

Sem1i-paraffin-base oil contains, in addition to paraffins and ole- 
fins, naphthenes (C,H.,). These compounds have the same chem- 
ical formulae as the olefins, but have markedly different character- 
istics. ‘The explanation for this is in the way that the C and H 
atoms are united, differing in the two series, the carbon particles in 
the olefins existing as a simple chain, whereas the naphthene car- 
bon atoms are considered as being grouped as a closed ring. This 
class of oil is found in southern districts, like Louisiana and ~ 
Oklahoma. 

Asphalt- or naphthalene-base oils are made up largely of naph- 
thene and olefins, paraffins being almost entirely absent. Examples 


MANUFACTURE AND DISTRIBUTION OF GAS 305 


of this kind are found in Texas and California. The naphthene 
series are much more stable than the paraffin; they do not yield 
paraffins or olefins in cracking under heat, but pass at once into 
members of the benzene series, such as benzene, toluene, xylene and 
higher members. These benzenes exist in the gas only as vapors, 
and are subject to the laws of vapors regarding saturation and 
precipitation; consequently, gas made from naphthene oils must 
be very carefully handled in the processes subsequent to generation 
to secure to the consumer equal candle-power at all seasons of the 
year. Tor additional information on the treatment of this gas I 
would refer you to a paper presented to the American Gas Insti- 
tute by W. H. Gartley in 1907. 

The results of gasifying the oil show that the various hydro- 
carbons evolved depend, as to nature and relative quantities, on 
time, temperature, relative quantities of oil injected, and amount 
of heat available from the fire-brick. The richer illuminants pre- 
dominate, of course, and this rich oil gas, mixing with the blue 
water gas, results in carburetted water gas, and which has a high 
candle-power and calorific value. By varying the relative quan- 
tities of blue gas and oil gas, and the heats, time of run, etc., the 
candle-power and heating value may be made high or low, as de- 
sired. The maximum and minimum limits would be about as 
follows: With no blue gas and all oil gas, the candle-power would 
be about 85, and the calorific value about 1300 B.t.u., or, with 
all blue gas, and no oil gas whatever, the candle-power would be 
practically zero, and the calorific value about 320B.t.u. Any 
intermediate condition could be attained, but in practice it is found 
that a gas exceeding 26 to 30 candle-power, burns with a smoky 
flame in ordinary burners, under usual conditions, and, further- 
more, the tendency of the present time seems to be towards a 
standard gas of an average of about 600 B.t.u. calorific value. 

When a water-gas set has been making gas for a certain number 
of minutes it becomes too cool for economical operation. ‘The oil 
is then shut off, next the steam, and then the stack valve is opened. 
Thereupon the blast is turned under the fire and the whole cycle 
of operations is repeated. 

On account of the steam striking the under side of the fire and 
cooling it too rapidly, it is now customary to make a so-called 
“down run” every third or fourth time. This simply means that 
the direction of flow of the steam through the fire is reversed, now 


306 ILLUMINATING ENGINEERING 


passing downwards instead of up, the connections on the machines 
being so arranged as to permit of this being done. As often as the 
fire requires it, fresh coke or coal is put into the generator, the 
ashes and clinkers being taken out at the bottom. 

From the description given of the principles involved in the 
Lowe process it will be seen that it is essentially an intermittent 
one. In the first place the deep fuel bed is brought up to a high 
temperature in the most economical way, namely, by internal com- 
bustion in the generator, and in this way differs from the early 
processes first mentioned. 

Secondly, it differs from the second type, or that promulgated 
by Tessie du Motay, in making use of the heat from the generator 
gases to vaporize and fix the oil. 

These differences may be seen from the basic claim of the Lowe 
patents, which, in brief, are as follows: 


Basic Claim Lowe Patent 


The apparatus consists of the primary gas generator A, super- 
heater D, heat-restoring stack I, boiler R, the usual washer V, and 
scrubber Y. 

The gas generator A is filled with anthracite or bituminous coal, 
air is forced by a blower through the heat-restoring stack I and 
pipe L into generator A below the grate bars, having been pre- 
heated in passing through stack I. 

The products of combustion are conducted from the top of 
generator A through pipe F, through the superheater, which is 
filled with loose fire-brick above the arch, to the atmosphere through 
stack I, Valves E’ and H having previously been opened. 

The heat from the out-going gas is partially transferred to the 
air from the blower, which is forced around the stack tubes into 
pipe L. After the fuel in the generator is thoroughly incandescent 
and the superheater is heated, the air is cut off and the valves E 
and H are closed. 

Steam is now admitted into the top of the superheater through 
K’ from boiler R. 

The steam in passing through superheater becomes intensely hot, 
and is admitted to the generator below the grate bars through 
pipe H’. The steam in passing through the heated carbon is de- 
composed, liberating hydrogen and producing a proportionate quan- 
tity of CO,. The CO, in passing through the heated carbon is, for 


307 


MANUFACTURE AND DISTRIBUTION OF GAS 


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308 ILLUMINATING ENGINEERING 


the most part, changed to CO, and the gas at the top of the fuel 
bed is H, CO and a small part of CO,. 

At the same time steam is admitted to the superheater, petroleum 
or other hydrocarbon oils are introduced in regulated quantities 
from tank M on to the top of the hot coals in the generator, where 
it is volatilized and mixes thoroughly with the gas coming through 
the fuel bed. These gases are then fixed by the heat before leaving 
the generator from which they pass to the top of the boiler R 
through numerous tubes, transferring some of their sensible heat 
to the water. All of the steam used for the gas-making process is 
furnished by this boiler, and the heat of the gas is the only energy 
used for generating the steam. 

Passing through the boiler the gas enters the washer V, thence 
through the scrubber Y into the purifiers, and finally into the 
holder A’. 

It should be stated that this apparatus never worked satisfac- 
torily for the reason that the oil gas was not subjected to suffi- 
cient heat to fix it into a permanent gas. Mr. Lowe later changed 
his method, although conforming to the original patent, and sub- 
stituted in place of the superheater for drying and superheating 
the steam, a superheater filled with checker-brick properly heated 
by internal combustion in the superheater of the producer gases 
formed in the generator at the time of blasting up the heats. When 
making gas the blue water gas from the generator, with the oil 
vapors generated at the top of the generator, pass through the 
superheater for the purpose of fixing the oil vapors; this principle 
being the same as that employed in all water-gas-making appa- 
ratus up to the present time. 

Returning for a moment to the original table giving composition 
of gases, it may be stated that the illuminants methane and ethane, 
result from gasifying the oil, while the carbon monoxide and hydro- 
gen result from the action of steam upon the incandescent fixed 
carbon in the generator fuel. The balance of the constituents re- 
sult from both sources, but to a varying extent. 

The subject of the efficiency of a Lowe water-gas set as a heat 
machine may be stated practically about 60 per cent. The subject 
is too lengthy to be discussed here, but anyone interested is referred 
to a paper by Mr. A. G. Glasgow, Proceedings American Gas Light 
Association, 1890, or to an abstract thereof which appears in the 
“ Mechanical Engineers’ Pocket-Book,” by William Kent, under 
the general subject of illuminating gas. 


309 


MANUFACTURE AND DISTRIBUTION or Gas 





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310 ILLUMINATING ENGINEERING 


In general, we may use the following average figures to illustrate 
the efficiency of a Lowe water-gas set, all per thousand cubic feet 
gas made reduced to 60° F.: 


Pounds anthracite: generator fuelic. 12) eee 30 
Pounds 01] admittedto* carbureter: =: {oyneen 32 
Pounds steam used during run............... 30 
Pounds resulting gas produced............... 46 


This serves to introduce the principles of water-gas manufacture, 
and we will now discuss the subject of treatment of this gas after 
leaving the generating apparatus. 


Purification of Water Gas 


Coming to the subject of impurities, we find that tar and sulphur 
are the predominating ones that must be abstracted. Before treat- 
ing these, however, it is to be remarked that water gas is to be 
condensed, in a measure, similar to coal gas. Water gas, however, 
in modern practice, is not reduced to as low a temperature in the 
works as is coal gas. 

The principle of water-gas condensation, however, is the same 
as for coal gas. The heat to be abstracted consists of the sensible. 
heat plus the latent heat of vaporization of the various gases and 
vapors which compose the gas. This results in deposition of some 
of the heavier hydrocarbons, forming the so-called water-gas tar. 

In modern practice water gas is seldom condensed below 90° F., 
because its purification is most economical at this or somewhat 
higher temperatures, and also because more of the richer illumi- 
nants remain in the gas at the higher temperatures. A large amount 
of condensation takes place in the relief holder. 

Naphthalene is easily avoidable in water-gas practice by proper 
regulation of the heats. 

Tar is extracted from water gas by condensation, washing and 
scrubbing, and also by mechanical means, such as a P. & A tar 
extractor. With the oily water-gas tar, however, the P. & A. must 
be operated, between rather narrow limits of temperature, say be- 
tween 105° and 110° F., and under great differential pressure. 

Usually, after all the washing and scrubbing, there remains a 
mist of light tarry vapors which are exceedingly difficult to ex- 
tract. This is perhaps best accomplished by means of shaving 
scrubbers, in which light wood shavings simply absorb the mist 
as the gas slowly passes. 


MANUFACTURE AND DISTRIBUTION OF Gas 311 


Sulphur exists again as H,S and organic sulphur, and is usually 
removed by means of iron oxide as described under coal gas. In 
coal gas the purification is usually carried on under lower tempera- 
tures than in water gas, because in coal gas the gas is previously 
reduced to a low enough temperature to permit the extraction of 
the ammonia. 


Carburetted Water Gas as Made from Oil and Steam Only 


Lowe Oil Gas. There is time here only for a brief mention of 
carburetted water gas as made from oil and steam only. This 
process is more largely used on the Pacific slope on account of 
the low cost of oil and the high cost of coal and coke. 


Jones Ore Gas Set 





The development of this oil-gas process is due to the efforts of 
Mr. Lowe, as well as largely to Mr. E. C. Jones, Chief Engineer 
of The San Francisco Gas & Electric Company. 

The principles underlying the manufacture of gas by this method 
are unique in a way. No standard type of apparatus has been de- 
veloped, but there are various forms of one-shell and two-shell 
types in use on the Pacific coast to-day. 

These shells are of iron, lined with fire-brick and checkered with 
fire-brick. ‘To heat up the set oil is introduced, or sprayed in with 
a steam spray, and burns by means of an air blast, the products of 
combustion passing off through a stack valve in the usual manner. 


312 ILLUMINATING ENGINEERING 


When the set is up to heat the air blast is cut off, and the oil and 
steam admitted alone. An accurate adjustment of the quantity 
of oil to the heat is necessary for best results. | 

The oil gasifies under the heat of the fire-brick, and the steam 
is partially decomposed into its elements. Some of the heavier 
illuminants are decomposed, and considerable free carbon or lamp- 
black results. The gas produced, as will be seen from the early 
tables, resembles coal gas very much in its analysis. 

The impurities to be removed from this oil-gas process are lamp- 
black, tar and sulphur. The lamp-black removal, handling and 
treatment is a problem in itself, but it is removed from the gas 
by washing with copious quantities of water, and by scrubbing, and 
is subsequently fired under the boiler.in a wet state, or it can be 
used as generator fuel in an ordinary water-gas set. 

The tar and sulphur are removed in the customary ways. Oil 
gas, as made above, is treated much like ordinary water gas, except 
it is never passed through condensers, but is subjected to much 
washing and scrubbing. ‘This process of treatment at once appeals 
to anyone as being logical, on account of the large quantities of 
lamp-black made during its generation. 

Under conditions of best practice to-day, this process of gas 
manufacture requires about a total of 7 to 8 gallons of oil per 
1000 cubic feet made, and there is every likelihood that this quan- 
tity will be materially reduced. From general figures it would seem 
that only about 2 gallons of oil should be necessary to supply the 
required amount of heat, and if we figure an average of 41% gal- 
lons for making the gas, it would seem as though from 6 to 61% 
gallons will ultimately be all that is required for this process. Re- 
cent results indicate that these figures may be attained. 


Producer: Gas 


Producer Gas. Producer gas is usually made by one or both 
processes already explained under coal- and water-gas manufacture. 
In some forms it consists of CO and N, produced by air being 
blown through a bed of incandescent fuel, the resultant gas having 
a calorific value of about 120 to 130 B.t.u. per cubic foot. If, in 
addition to air, we add steam, the resultant gas will contain H, 
CO and N. If steam alone is used the gas will consist of H and 
CO, and will have a heating value of about 320 B. t. u. 

Gas, as an agent for the production of light and heat, must not 
be understood to be restricted to artificial gas, as before outlined, 


MANUFACTURE AND DISTRIBUTION oF Gas AL 


but many other forms besides these mentioned are used, such as 
retorted oil gas, blast-furnace gas, acetylene, gasolene air gas, 
resin gas, wood gas, hydrogen-methane gas, garbage gas, ete. 

Producer gas is only mentioned at this time on account of its 
adaptation to gas-engine practice. 














































































































































































































STATION MeTER Drum. 
pres GW 


Metering Gas at Works—The Station Meter 
Station Meter. ‘The gas after passing through the purifiers is 
ready to sell, except that the amount made must be determined 
in order to keep the several parts of the works under control. This 


314 ILLUMINATING ENGINEERING 


measuring is usually done by means of a large four-compartment 
drum which revolves in a cast-iron case filled about two-thirds 
full of water. 

The inlets and outlets of the drum compartments are so ar- 
ranged that when the outlet is below water the inlet is above, and 
the compartment fills with gas. The drum revolves something like 
a squirrel cage, and shortly after the inlet dips below the water the 
outlet comes above and the compartment discharges its contained 
gas. The cubical contents of the compartments being accurately 
known, the motion of the drum is communicated by gearing to 
the dial, and thus we have an apparatus which accurately measures 
the gas made. It is customary to make proper corrections for tem- 
perature and barometric pressure, and in practice we reduce the 
gas manufactured to a basis of ‘60° F., and 30 inches barometric 
height. 

On account of the large size of station meters various forms of 
proportional meters have been tried. ‘These measure only a small 
fraction, usually 1 per cent, of the make, and are also arranged to 
register the total, but so far there is really no reliable proportional 
meter on the market for measuring artificial gases. 

Recently various other methods of measuring gas have been 
tried. Drums have been made of the rectangular screw-thread type, 
rotary meters have been introduced, and the most recent is the 
electric gas meter. The time is too limited to attempt to explain 
these in detail. 


Gas Holders 


Gas holders are simply inverted cups placed in water, and so 
arranged that the gas enters or leaves the holder above the water 
through pipes arranged for the purpose. They act as storage 
reservoirs for gas, and thus allow the plants to manufacture uni- 
formly during the 24 hours, taking care of constantly varying con- 
sumption. ‘The only principles involved are as given, and the 
great questions involved in connection with gas holders, outside 
of their design and construction, are “ How much gas-holder ca- 
pacity is required as related to the maximum manufacturing ca- 
pacity?” and “ Where shall these holders be located—at the gas 
works or in other localities? ” 

The latter question is largely a matter of distribution methods, 
and the former the question of the minimum permissible holder 


MANUFACTURE AND DISTRIBUTION OF Gas 315 


capacity any plant may have and be safe. This is a very important 
engineering and commercial detail. 


Istribution 
In the distribution we have a vast subject, and one in which 


many problems remain to be solved. 

Formerly gas was sent out from the works at not to exceed the 
maximum pressure thrown by the works holders. In such systems 
the delivery obtainable from a given size and length of pipe was 















iw 


aw. 


al 
aA iD 
eae 


LOD 





vara 


ar arate aaa aa 









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RID LI PIPL A 




















SSC 


SSS 








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~SECTION~ 


Fic. 18. 


that due to the differential head or pressure between the lowest 
permissible pressure at the extreme end of the pipe, say 2 inches 
water column, and the maximum holder pressure, which rarely ex- 
ceeded 5 or 6 inches. Thus the actuating pressure, or differential 
head, was very little, possibly less than one-tenth of 1 pound per 
square inch. 

Subsequently, as cities spread out, gas holders were erected in’ 
outlying localities and called district holders, and these supplied 
the district surrounding them, as before, by low pressure. ‘These 
district holders were filled with gas through separate pumping 
mains from the works. 

In the course of time these systems were found to be inadequate, 
and if re-enforced under the low-pressure ideas would have entailed 


12 


316 ILLUMINATING ENGINEERING 


vast construction expenditures to remedy conditions sufficiently 
to produce good service. 

To overcome bad distribution systems, especially in the larger 
cities, the next step in the progress of distribution methods was to 
erect pumping plants at the works and holders, run separate pres- 
sure re-enforcing pipe lines to the heavy points of consumption, 
and there install some device to automatically reduce the pressure 
to the required regular distribution pressure. In some cases pres- 
sure-indicating instruments were located at such points of heavy 
consumption, and no regulators used, the pressure and amount of 
gas pumped being controlled at the works and holders so that a 
given pressure was maintained at this point where the indicator 
was located. The instruments transmitted the amount of pressure 
back to the pumping plant, or a small separate-pressure tell-tale 
line was used. ‘These systems used various pumping pressure, 
usually not over 5 pounds per square inch. — | 

In the meantime still another development was taking place. 
Communities were growing and spreading out in all directions, and 
especially around the larger cities where suburban communities 
were being formed at some distance from the cities, and in which 
the houses were far apart. It was not possible to profitably supply 
such places with gas with the great investment required in low- 
pressure mains under the old system, so high pressure was devel- 
oped to meet this requirement. 

Pressures up to 50, 60 and even 80 pounds per square inch are 
now being used, as compared to the old system of low pressure, 
with a maximum of about 14 pound. Such high pressure requires 
reduction to say 4 or 6 inches water column before entering the 
piping in the consumer’s building, and this is accomplished by 
automatic gas regulators, a number of different types of which 
are now on the market. 

Reasons for High Pressure. In the meantime other forces were 
at work tending to hasten the advent of high pressure. The uses 
to which gas was applicable were increasing in number, it was 
also used more freely in hghting, heating and power work, and 
this resulted finally in very much larger sales of gas per capita 
per annum than prevailed formerly. Add to this the fact that 
the price of gas was gradually being reduced and another stimulus » 
is seen. ‘Thus vast quantities of gas were being consumed as com- 
pared to former years. 


MANUFACTURE AND DISTRIBUTION OF GAS 317 


The principles underlying the development of high pressure then 
resolve themselves into the fact that it was necessary to provide 
for vastly increased demand, and also that the distances through 
which it was necessary to supply gas in large quantities were greatly 
augmented. 

This development was not rapid in the early days of the gas 
business, and, in fact, it may be said to have developed with the 
advent of fuel gas, the possibilities of which have only been realized 
within the most recent years. In fact, it may be said that the 
application of high pressure to artificial gas is a development of 
the last decade. 

Higher pressure permits of small pipes to transmit large quan- 
tities of gas. The reasons for this are that the differential head 
is very much greater than under low pressure, and also a given 
mass of gas occupies a much smaller space when compressed. 

The flow of gas, or any liquid through pipes, is governed by the 
differential head or effective driving pressure, the length of the 
pipes, its diameter, the condition of its interior surface, whether 
the line is straight or full of turns, the density of the traversing 
gas or fluid, and the questions of pulsations, obstructions, etc. 

Formulae for Pipe Conductivity. Various formulae have been 
devised to determine the flow of gas in pipes, but the one com- 
monly used for low pressure is Dr. Pole’s formula. 


Qae ,/th 


Q=quantity of gas in cubic feet per hour at atmospheric pressure. 

e=a factor, which may vary from 1000 to 1400, but a fair average 
value for which is:1250. This factor is inserted for the 
purpose of allowing for condition of the interior pipe sur- 
face, obstructions, such as tar, etc. 

d=diameter of pipe in inches. 

h=differential head, or pressure, in inches of water. 

s=specific gravity of gas, air being 1. 

l=length of pipe in yards. 


From this formula it appears that the capacity of a pipe to trans- 
mit gas under low-pressure conditions, among other factors, varies 
as the square root of the fifth power of the diameter. As a result 
of this it may be stated that when a pipe is doubled in diameter 
its capacity under low pressure is multiplied about 5.6 times. 


318 ILLUMINATING ENGINEERING 


For High Pressure. The following formula covers the range of 
high-pressure artificial gas: 
Q=33.3 jf Pera) 

Ls 

Q=quantity of gas in cubic feet per hour at atmospheric pressure. 
d=diameter of pipe in inches. 
p, absolute initial pressure in pounds per square inch. 
p.=absolute terminal pressure in pounds per square inch. 
L=length of pipe in miles. 
s=specific gravity of gas, air being 1. 








For Very High-Pressure and Long Pipe Lines. ‘The formula for 
ordinary high-pressure work, previously given for use with artificial 
gas, is found to give results that are too small when applied to a 
higher range of pressure and long pipe lines. In particular, for 
natural-gas work, where pipe lines many miles in length are in 
use, it is found in practice that more satisfactory results are se- 
cured from the following formula: 


2 2 
Q=42a / Ps =p," 
iv 


Q=quantity of gas in cubic feet per hour at atmospheric pressure. 
a=a factor, which in practice is found to vary with the diameter 
of the pipe, and for which fairly satisfactory amounts have 
been determined. For instance, a=95 for a 6-inch pipe, 
556 for a 12-inch, etc. See Ohio Geological Survey report. 
p, absolute initial pressure in pounds per square inch. 
p.=absolute terminal pressure in pounds per square inch. 
L=length of pipe in miles. 

This last formula is based upon a gas of 0.6 specific gravity. 
Where the gravity of the gas varies the quantity found is multi- 
plied by the square root of 0.6 divided by the gravity determined. 
Temperature corrections are usually neglected in natural-gas 
measurement. 

Elevation. In the olden days the question of elevation was 
pertinent. Gas, being lighter than air, in a confined pipe tends 
to exercise greater pressure at higher elevation, as compared to 
the atmosphere, because it weighs less than the equivalent column 
of air under the condition of being exposed to atmospheric pres- 
sure at the initial low point, as, for instance, through a gas holder. 


MANUFACTURE AND DISTRIBUTION OF GAS 319 


When gas was distributed entirely under low pressures some points 
of a given city lying much below the level of the works received 
insufficient pressure, and other points, much above the works, re- 
ceived excessive pressure. 

Recently, however, where high pressure is used, the question of 
elevation causes no concern because of its comparatively slight ef- 





Fic. 19.—Station Governor. 


fect under such conditions. Under low-pressure conditions, and. 
with gas of about six-tenths specific gravity, the difference in pres- 
sure due to 100 feet elevation is about six-tenths inches water 
column. 

Station Governor. A station governor is an apparatus which 
automatically maintains a given outlet pressure, which must be 
less than the inlet pressure. This is simply produced by the effect 


320 ILLUMINATING ENGINEERING 


of the outlet pressure on a float or a diaphragm. Some governors 
have been devised which increase or decrease the pressure auto- 
matically according to the demand. 


Principles of Design of a Distribution System 

We will first consider the principles underlying the design of 
a low-pressure distribution system. 

Under this kind of a system we are limited to the maximum 
pressure allowable on consumers near the plant or holders, and by 
the minimum pressure allowable on the outlying consumers. For 
purposes of illustration, assume this maximum and minimum to 
be 6 and 2 inches, respectively. Then the maximum differential 
head is 3.8 inches, allowing 0.2 inches drop in services. 

Next, having a complete map of the city, it is necessary to deter- 
mine the maximum demand per unit of area, which for purposes 
of illustration we may assume as 1 square mile, and having selected 
the center of each square mile, we proceed to run low-pressure 
feeders from the works, in several directions if necessary, and large 
enough to furnish all the gas required at peak load to each unit 
of area reached by such main, and under the limitations of pres- 
sure assumed. If we determine that the loss of pressure from 
the center of each unit of area, to the outside limits thereof at 
peak load, shall not exceed 1 inch, then the maximum drop in pres- 
sure in the feeders from the holder outlets must not exceed 2.8 
inches to come within our assumed limits. 

On the basis of this assumption we are thereupon obliged to 
design the distribution system in each unit of area so that at peak 
load the maximum drop in pressure from the center, or point of 
supply from the feeder mains, to the farthest outlying point in 
each area shall not exceed 1 inch at peak load to come within our 
required assumed conditions. 

To do all this requires the knowledge of maximum demand per 
consumer, the probable maximum number of consumers per block 
and per unit of area, the length of blocks, and certain other prac- 
tical considerations, such as presence of electric surface-car line 
tracks, ete. 

Naturally, smaller and simpler systems for smaller cities are 
easier to design, but the principle of maximum permissible drop 
in pressure is the same. 


ie 


MANUFACTURE AND DISTRIBUTION OF Gas 321 


Design for High Pressure. When we come to consider high- 
pressure systems the same general principles hold true. We may 
run high-pressure feeders to the centers of the units of area, or 
we may design them to carry only moderate pressure, say up to 
5, 6 or 8 pounds per square inch. If such a system is adopted, 
it becomes necessary to install pressure-reducing devices at the 
points where the high-pressure feeders deliver gas into the low- 
pressure system. Such devices are called district regulators. 





== 





Fic. 20.—Section of Manhole on 5-lb. High-Pressure Line. 


Another entirely different system is to carry moderate or high 
pressure on the entire system of mains. In such cases it is neces- 
sary to install pressure-reducing devices on each pipe entering each 
and every consumer’s premises to reduce the main-pipe pressure, 
whatever it may be, to the pressure required by the consumer. 
Such devices are called individual gas-pressure regulators or gov- 
ernors. 

The advantage of the use of high pressure lies in the fact that 
much smaller distributing pipes can be used, thus saving great 
investment charges. The cost of compressing gas is generally a 
small item. 


322 ILLUMINATING ENGINEERING 


Drainage of Mains. Artificial gas, as it leaves the works, always 
contains water vapor and various hydrocarbon vapors, which con- 
dense out of it as it passes through the distributing pipes, owing 
to changes of temperature and other causes. These vapors con- 
dense and liquefy, forming the so-called drip water. For this 
reason it is necessary to lay artificial gas pipes on a slight grade, 
and at the low points devices for collecting this drip water are 
installed so that it may be pumped out. 





Fig. 21.—High-Pressure Main, Service Meter and Drip Installation. 


A. %” saddle with 5/16” main top L. 1’ ell. 
(galvanized). M. 1’ vent from safety seal (end 
B. %” corporation cock with 14” protected with No. 16 wire 
opening. gauge). 
C. 3,” street tee (galvanized). N. 3%” ell. 
D. %” street ell (galvanized). O. Reducing ell. 
E. 3%,” curb cock with full gas  P. Reducing ell. 
way. Q. 1” ell. 
F. %” street ell (galvanized). R. To riser. 
G. 34” tee (galvanized). S. Mercury seal (see schedule). 
H. 34” meter cock with 5/16” gas T. 1” x 3@” tee. 
way. U. %” long screw. 
J. High-pressure governor (see V. %” ell. 
schedule). W. %” vent from regulator. 
K. Cross. xX. 1” ell: 


MANUFACTURE AND DISTRIBUTION OF GAS Bo 


Materials and Joints. For low-pressure distribution, and in the 
larger cities it is customary to use cast-iron for the main pipes 
on account of its long life, resistance to corrosion and to electrolytic 
action. ‘The joints are almost universally of the bell and spigot 
type, in cast-iron mains, and the jointing material is either lead 
or cement, caulked or placed into the joint against an inside roll 
of jute packing to prevent it from entering the pipes. 

Such joints are not conceded to be safe at high pressure, and 
when wrought-iron pipe is used screw or threaded joints are used. 

On account of the mechanical strength of cast-iron, it is to-day 
the general practice to use but little pipe smaller than 4-inch cast- 
iron pipe for gas distribution, so that under that size wrought pipe 
is employed. Under 6-inch pipe the wrought is usually cheaper 
in first cost than cast-iron pipe. Pipes are usually much stronger 
than required to merely resist the internal pressure. External con- 
ditions, such as pressure of soil, loads, settlement, corrosion, etc., 
are the factors which determine the minimum permissible thickness 
of pipes. 

Special types of pipes and joints have at various times been 
brought forth, such as Universal, vitrified clay, and even wood has 
been used, but to a very small extent. 

Gas mains are usually laid deep enough to be under the frost 
line, and are kept away from car tracks and underground obstruc- 
structions as much as possible. Services, or the pipes leading from 
the mains to the consumers’ premises wherever possible, are graded 
into the mains. 

Electric surface-car lines have proved a bug-bear to underground 
piping systems on account of electrical current leakage setting up 
an electrolytic action. A portion of the return current from such 
car-line systems finds its way into the piping and leaves it again 
usually at some point near the generating or substations, or where 
it jumps to some other conductor. The troubles occur where the 
current leaves the pipes. 

Various remedies have been suggested and tried, such as double. 
systems of piping, one on each side of the car tracks, also various 
forms of insulated pipe covering and joints, also bonding the pipes 
to the rails or to the return conductors. All, so far, have proven 
to be more or less in the nature of palliatives and not complete 
remedies. The subject of electrolysis is one of great importance. 

As I have used more than the time allotted me, I shall not take 


324. ILLUMINATING ENGINEERING 


up the subject of the gas meter, the instrument employed for 
measuring the amount of gas used by the consumer, or house piping 
or photometry, as I understand some of the subsequent lectures 
will incorporate about all there is to be said upon these subjects. 

I would like to say a few words regarding calorimetry. Owing 
to the fact that by far the greater proportion of gas sold to-day 
is sold as a heating agent, either through fuel appliances or through 
mantle burners, it seems necessary to change our system of meas- 
uring quality to one that will define the calorific value. This may 





Fig. 22. 


be determined in two ways, first, from the chemical analysis gas, 
as the heating value of its constituents are pretty well known. 
There has been, however, adopted for quite general use an instru- 
ment whose essential principle of operation is, that the products 
of combustion of a gas shall be passed through a vessel which is 
water-jacketed, and in which the radiated heat from the flame and 
the sensible heat from these products of combustion are absorbed 
by water in the jacket. The quantities of gas and water being 
known, the rise in temperature furnishes a measure of the amount 
of heat liberated by the combustion of that amount of gas. 


VII (2) 
THE MANUFACTURE AND DISTRIBUTION OF ARTI- 


FICIAL GAS, WITH SPECIAL REFERENCE TO 
LIGHTING 


By Water R. AppicKs 


Introduction 

The subject of this lecture, ‘“‘' The Manufacture and Distribution 
of Artificial Gas, with special reference to Lighting”, is so com- 
prehensive that it is difficult to outline the field without shghting 
essential features of the gas business covered by the assigned 
subject. 

The following sub-divisions are made to facilitate reference. 

(A) Quality of Artificial Gas. 

(B) Purity of Artificial Gas. 

(C) Uses of Artificial Gas. 

(D) Kinds of Artificial Gas (including Natural Gas for com- 
parison ). 

(E) How Artificial Gas is manufactured. 

(F) The handling, within the gas plant, of raw materials, of by- 
products, and of the finished product, Artificial Gas. The Retort 
Coal Gas Process described for illustration, with some reference to 
an auxiliary carburetted water gas plant useful for enriching coal 
gas, for utilizing the coke by-product of the Retort Coal Gas Plant, 
and caring for variation in the daily demand for gas. 

(G) Distribution of gas from Storage Holder at Plant through 
transfer mains to the City Distribution Holder. 

(H) Distribution of gas ‘from Distribution Holder through 
Street Main System to the gas service pipes leading to the houses. 

(I) Distribution of the gas from the Street Mains through gas 
service pipes, house service pipes, meters and governors to appli-- 
ances for utilizing the gas. 

(IK) Observations relating to the piping of modern buildings and 
its relation to other utilities in use. 

(L) Observations relating to the appliances used in burning gas. 

(M) Influences that govern, in the selection of a particular type 
of gas, in a given geographic location. 

(N) The future of the Artificial Gas business. 


326 ILLUMINATING ENGINEERING 


A. Quality of Artificial Gas 

Gas should no longer be manufactured with special reference 
to lighting alone; it must still be designated by its candle power, 
where State laws, special and general, define quality as the candle 
power given by a specified quantity of gas burned through a flat 
flame or argand burner. The same quantity of gas burned by 
means of a bunsen burner as a heating flame in contact with the 
Welsbach mantle will give four times the light. It is quite common, 
in describing an artificial gas, to say that it is a 16, 18, or 20 
candle power gas, meaning that when a specified quantity of gas is 
burned in a specified burner that it will give 16, 18, or 20 units of 
light when compared with the original unit of light, the candle. 


B. Purity of Artificial Gas 
It is required in many States that manufactured gas shall be 
free from sulphuretted hydrogen, and contain but limited quan- 
tities of ammonia and fixed sulphur. Such laws are quite unnec- 
essary for the reason that the extending use of electricity will com- 
pel commercial purity in gas. 


C. Uses of Artificial Gas 


Artificial gas is used for :— 

(la) Lighting by means of the flat flame or argand burner. 

(1b) Lighting by means of heat generated by the gas when 
burned in a Bunsen burner to a blue flame and making incandes- 
cent the fabric of the gas mantle. 

(2) Heating through the use of the Bunsen flame in gas ranges 
for cooking, in a multitude of industrial appliances increasing day 
by day, and in steam boilers. 

(3) Power by means of the internal combustion engine, made 
familiar to all by the introduction of the automobile. 


D. Kinds of Artificial Gas (Including Natural Gas for 
comparison ) 


Artificial Gases are known as:— 

(1) Water Gas, an odorless gas, containing Hydrogen and Car- 
bonic Oxide, giving a non-luminous flame when ignited; is no 
longer distributed. It must not be confused with 

(2) Carburetted Water Gas which is a mixture of water gas and 
oil gas having a distinct and pungent gas odor, and when burned 
gives a brillant white flame. 


MANUFACTURE AND DISTRIBUTION OF GAS Bat 


(3) Retort Coal Gas, a gas of lower specific gravity and less 
brilliant flame than Carburetted Water Gas. 

(4) Coke Oven Gas, similar in all respects to Retort Coal 
Gas; only 35% to 50% of the gas made is distributed, the portion 
distributed is usually of equal candle power to Retort Coal Gas. 
The remainder of the gas is burned under the ovens in place of coke. 

(5) Oil Gas, a heavy petroleum gas which when burned in prop- 
erly constructed burners gives a bright light. The California Oil 
Gas distributed on a large scale in California is a type of this gas. 
The familiar Pintsch Gas used in railroad passenger cars is a type 
of this gas: Blau Gas is another. Carburetted Water Gas contains 
from twenty-five to forty per cent of oil gas. 

(6) Acetylene Gas gives a brilliant white light when properly 
burned. It is prepared as required by adding water to calcium 
carbide: the lamps of automobiles are a familiar example of its 
use. In country districts, hamlets, villages and small towns are 
supplied from a central plant with this gas. 

(7) Carburetted Air Gas. This gas is the familiar type used in 
country houses and hotels; it is simply air saturated with vapors 
of gasolene. 

(8) Producer Gas contains Nitrogen and about 25 per cent com- 
bustible gases; when cold usually requires heating to make it 
ignite; is seldom distributed beyond the boundaries of a manu- 
facturing establishment. 

(9) Natural Gas, one coming from the earth usually in a dis- 
trict where petroleum oil is also present, and frequently under 
pressure of many atmospheres; it is usually sold at much less cost 
than artificial gas so long as the natural gas supply remains 
available. 


H. How Artificial Gas is Manufactured 


(1) Water Gas, sometimes called blue gas, is made by raising 
the temperature of a fuel bed, by means of a forced blast of air, to 
incandescence (the Producer Gas made usually being wasted), when, 
the air being shut off, steam (H,O) is passed through the fuel bed 
(C,), which, on decomposing yields Hydrogen and Carbonic Oxide 
(CO), the Carbon being supplied by the fuel. Usually hard fuel 
is used, either anthracite coal or coke, though bituminous coal has 
been used. The fuel bed is usually contained in a cylindrical shaped 
fire brick furnace (Fig. 1 illustrates a twin generator) which in 
turn is surrounded by a gas pressure tight cylindrical steel rivetted 


328 ILLUMINATING ENGINEERING 


shell, supplied with gas tight stack valve, coaling and cleaning 
doors aud proper air, steam and gas connections governed by valves, 
all manipulated by the gas maker. ‘The cylinder containing this 
fuel bed is commonly called a Generator; when single it is eight to 
twelve feet in outside diameter and twelve to twenty feet in height. 
Water gas is colorless, odorless, specific gravity .550, yields on 
analysis (Stillman) CO, 0.14, O, 0.13, illuminants 0.0, CH, 7.65, 
CO 37.97, H, 49.32, N, 4.79; on burning yields only a blue, non- 
luminous flame and 385 B.t.u. per cubic foot. 


CARBURETER 

















ji | bn tf 
Re. 

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DOUBLE CENERATOR 
Fie. 1. 


(2) Carburetted Water Gas: The water gas constituent of this 
gas 1s made in an identical manner as above outlined for water 
gas. The oil gas constituent may be made by heating oil in ex- 
ternally heated retorts, but is now usually made as follows: (2) The 
cylinder described for making water gas is connected with sim- 
ilar cylinders in duplicate or triplicate, though the diameter 
may be slightly varied and the height is frequently increased by 
fifty per cent. The additional cylinders are not used for containing - 
fuel but are filled with many hundreds of standard fire brick placed 
in checker work fashion thus providing interstices between bricks 
for the passage of gases. The checker-brick work is raised to in- 


329 


AS 


a 


MANUFACTURE AND DISTRIBUTION oF G 









SER 











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TMG 503: 








Ate (eb oe 


330 ILLUMINATING ENGINEERING 


candescent heat by burning up the Producer Gas (wasted in the 
manufacture of water gas) made from the fuel bed of the Water 
Gas generator when it is being brought to incandescence by a blast 
of air preparatory to making water gas; all additional air required 
for this secondary combustion comes from the same source as for 
the first. When two additional cylinders are used the second is 
called the carbureter, because petroleum is dropped on the hot 
bricks in this cylinder and on vaporizing gives light-giving proper- 
ties to the water or blue gas flowing over hot from the Generator, 





Fic. 4. 


while the third cylinder, usually taller than the carbureter, is 
called the Superheater or Fixing Chamber; the function of the 
hot fire brick in this cylinder being the further heating of the 
water gas-oil gas mixture and the “ fixing ” of the oil vapor prod- 
ucts into fixed gas. This term fixed gas is used in a limited sense to 
include only usual atmospheric temperatures and pressures. (3) 
Carburetted Water Gas has the familiar pungent “ gas” odor; it 
has a specific gravity of about .660, contains normally as sold, no 
sulphuretted hydrogen, no ammonia and but seven grains of sulphur 
compounds in 100 cubic feet. It yields on analysis approximately 


331 


MANUFACTURE AND DISTRIBUTION OF GAS 


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q 


LLLLZ 


1 Section Through Retorts. 


Longitudina 


Fic. 5.—Bench of Gas Retorts. 


13 


B32 ILLUMINATING ENGINEERING 


(Mass. State Inspector 1897) CO, 2.91, O, 0, Lluminants 14.92, 
Marsh Gas 25.90, CO 25.30, H, 27.87, N, 3.04. ‘Ten thousand 
cubic feet of gas would require in its manufacture about 400 lbs. 
of fuel (where waste heat boilers are not placed after the car- 
bureters) and 446 gallons of oil; the gas produced would be about 
25 candle power and as a by-product may yield from 14 to 9/10 
gallons of water gas tar. 

(3) Retort Coal Gas is made by distilling at about 2200°-2600° 
Fahrenheit as much as 1000 lbs. of bituminous gas coal in a (4) 
clay retort having a “D” cross section typically 16 inches by 26 
inches and 9 feet to 20 feet long, either vertically, inclined or hori- 
zontally placed. The dimensions as well as the position may vary 
and the weight of charge is graduated to the retort capacity; i- 
variably the retorts are externally heated, (5) usually in groups of 
six to nine, by a single furnace but when retorts are 20 feet long, 
usually by two furnaces. Furnaces are usually fired without forced 
blast: The coke fuel is obtained hot from one of the group of 
retorts at the end of the distillation period, which varies from four 
to nine hours. About 10,000 cubic feet of gas of 16 to 18 c. p. 
is obtained from one gross ton of coal and there remains as by- 
products of manufacture, about 1000 lbs. of coke, about 12 gallons 
of tar, and ammonia sufficient to produce 20 to 22 pounds of sul- 
phate of ammonia. Retort Coal Gas in all essentials has the 
odor of carburetted water gas, though the manufacturer may dis- 
tinguish in the odor; it has a specific gravity of .400 to .450, 
and as distributed contains no sulphuretted hydrogen, though often 
12 or more grains of sulphur compounds, 0.3 grains of ammonia, 
and on analysis yields approximately CO, 1.75, O, 0, Iluminants 
4.88, Marsh Gas 33.90, CO 6.82, H, 46.15, N, may at times be 
found as high as 6.50 though 1.5% may be considered a fairer per- . 
centage. The heat units approximate 600. 

(4) (6) Coke Oven Coal Gas is manufactured by charging 
several tons of bituminous gas coal in the top of an elongated 
“D” oven 26” wide, 72” deep, and 30 feet long, and distilling it 
normally at a lower temperature than in the case of Retort Coal 
Gas but for periods varying from 24 hours to 36 hours. The heat 
for distillation is obtained by burning the poorer quality of -gas 
which comes off after the first 10 to 12 hours and, after removing 
the ammonia and tar, is supplied to the exterior of the coke oven 
through pipes at low pressures; air for combustion is in some sys- 
tems heated in regenerators by the waste combustion gases from the 


MANUFACTURE AND DISTRIBUTION OF GAS 320 


ovens and is supphed to the ovens under moderate fan pressures 01 
by the natural draft of tall stacks. This process is really not a gas 
making process but a coke making process, 1n which gas is but a by- 
product. 314 gross tons of coal produces 5200 lbs. of Coke similar 
to Bee Hive Coke and as by-products, 10,000 cubic feet of gas 








Un 


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NESS Wg 4 





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\V J or | NS NG SEG 
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y d SIRES, 


Longitudinal Sections. 


HERD © o 0 Ob tf 
Lt Jk Teakzic 


LA LA 


NS WTA 
OOS “Wiis “a ‘ZZ 


ee SSSI SS 


ST SAR MSS 





’ 
| 
: 
Y n 
Aa Ata 
Wr Gree SIDELELILEETLOLELEEEEETEEEEE EE —e — So cee 
SS Shhh bbb Dh hha Daas | | 


Section Through Regenerator. Elevation. Transverse Sections. 


Fic. 6.-—Early Type of Otto-Hoffman By-Product Coke-Oven. 


of 17 to 19 candle power, 30 gallons of tar, and ammonia suffi- 
cient to manufacture 73 lbs. of sulphate of ammonia. The by- 
product gas available for distribution has a specific gravity and heat 
unit value quite similar to retort oven coal gas and yields on analysis 
approximately CO, 0.1, O, 0.1, Illuminants 5.55, Marsh Gas 38.90, 
CO 6.57 (may reach 8.00) H, 42.1, N, 6.65, and when made with 


334 ILLUMINATING ENGINEERING 


sulphurous coals, contains as impurities, after purification with 
lime for carbonic acid, feul lime for fixed sulphur compounds and 
oxide of iron for sulphuretted hydrogen, normally 18 or more grains 
of sulphur compounds and 0.2 grains of ammonia. 

(5) Ow Gas may be made in iron retorts of similar pattern to 
the clay retorts used in Retort Coal Gas but they are smaller in 
cross section and usually not exceeding 9 feet in length: latterly 
clay retorts have been used. The external heating is effected by 
means of the best available fuel. As in Carburetted Water Gas the 
usual by-product is tar. Oil Gas burned in a special burner has a 
candle power of 60 to 100, specific gravity about that of air, and 
heat units of 1200 or over. (7) In California (see paper by 
K. C. Jones, American Gas Institute 1909, p. 410) Oil Gas is manu- 
factured on a large scale and by the use of specially designed appa- 
ratus, in which oil is used for fuel to heat up checker brick work in 
chambers quite similar to the carbureter and superheater of the 
carburetted water gas apparatus, as well as to make the gas. The 
character of the oil gas here made is distinct from oil gas made 
in retorts and for a comprehensive description the student is re- 
ferred to the able paper above referred to. The only residual, 
lamp black, is used in place of coal to manufacture water gas . 
which is mixed with the oil gas. The low labor charge per thousand 
cubic feet made is an argument for the use of this type of gas 
' where crude oil is very cheap. The analysis of the distributed gas 
is given as CO, 3.63, Illuminants 9.70, O, 0.34, CO 10.24, H, 
36.54, CH, 33.16, N, 6.39, Candle Power 21.88, B. t. u. 710.7 and 
specific gravity .523. 

(6) Acetylene is made by adding water to Calcium Carbide 
(which has previously been made in the Willson (8) or similar 
Electric Furnace from lime and coke). When burned in special 
burners the resulting gas gives an intensely brilliant white lght 
of about 250 candle power, has a specific gravity of .910 and a 
heat unit value of 756 B. t. u. 

(7) Carburetted Air Gas (9) is made by forcing air through a 
carbureter in such a manner that it will pick up 10 to 17 per cent 
of gasolene vapor. It must be burned in special argand or mantle 
burners. Its heat unit value has been given as 815 B. t. u. 

(8) Producer Gas is made in the Generator in the fuel heating 
period of Water Gas and Carburetted Water Gas manufacture. (10) 
It is likewise made by exhausting air or air and steam through 
any incandescent bed of fuel, or by means of a jet of steam below 


330 


MANUFACTURE AND DISTRIBUTION OF GAS 


O:x. Gas Ser. 


Jones” 


eerie 


Sy Seino RIN 





Fig. +s. 





ILLUMINATING ENGINEERING 


306 





Fia. 9. 





At DORON SI ONT LL RE: tt tee AER eee OTE TAA Ca ta EEE oss 








(alien eelaciap nimnonemsdaapacasatien pili AE lata sens ee 


Fiag. 10. 


MANUFACTURE AND DISTRIBUTION OF Gas Aro ae 


the ash pit injecting air and steam vapor through the fuel bed. 
This gas has a specific gravity of about .812 and heat units of 
135-165 B.t.u.. Containing about 75% N, for maximum efficiency 
it should always be burned in its hot state as it emerges from the 
generator, as in the case of Carburetted Water Gas manufacture. 

(9) Natural Gas has a specific gravity of .520 and heat units 1124. 
mraivers-CO. 0.0, 0, 1.3, Illuminants 0.5, CH, 95.2, CO 1.0, H, 
2.0, N, 0.0. It issues from the earth in some localities at a pressure 
of from 300 to 400 lbs. per square inch. Its high heat units and 
low price per thousand cubic feet make it, for any purpose, a com- 
petitor that artificial gas cannot compete with on equal terms. 
Natural Gas is here mentioned to accentuate this fact and to give 
a fairly complete view of the commercial field occupied by gas. 


F. Handling, Etc., Within the Gas Plant 


The only raw material required to manufacture simple coal 
gas by the Retort Process (or the Coke Oven Process) is a coking 
bituminous gas coal. In general, a first-class bituminous gas coal 
should contain at least 36% volatile matter, no more than 34 of 
one per cent of sulphur, and should be received in the condition that 
a 34” mesh screen at the mines would leave the larger portion 
of the run of mine coal. Natural conditions, handling, and cost of 
such a coal largely modify these general specifications. 

Gas Coal to-day is mined (lla) by Electric Mining Machines, 
is transported (11b) by pit wagons to the mine coal tipples (11c) 
and dumped (11d) into hoppers with screens, and if a gas plant 
is favorably located cars loaded (11le) with screened coal at the 
mines may be run into the Gas Works coal storage shed, or even 
into the retort houses and there unloaded into the retort house coal 
bins. In other cases coal cars may go to tide water and the coal 
be discharged into (12) large capacity ocean going steamers that 
will deliver the coal alongside the gas plant wharf several hundred 
miles distant, or the coal cars may be carried hundreds of miles 
and then discharge into harbor lighters of about 1000 or more. 
tons capacity which deliver the coal to gas plants five to forty 
miles distant. In the latter case the coal contained in the rail- 
road cars from the mines may be dumped bodily or through chutes 
into the lighters without any hand labor. (13a-13b.) The Astoria 
(14a) coal gas plant will serve as an illustration of retort coal gas 
manufacture: On arrival at this gas plant (14b) automatic grab 


O 
vA 
— 
cc 
— 
eS 
Z 
end 
ie) 
a 
= 
o 
Z 
= 
H 
a 
Z 
= 
= 
D 
| 
= 
— 


Fic. 11b. 





339 


MANUFACTURE AND DISTRIBUTION OF GAS 





Fic. 11d. 


340 ILLUMINATING ENGINEERING 


buckets picking up two gross tons of coal at a trip deliver the coal, 
through the instrumentality of an electrically driven traveling 
crane, to either 50 ton railroad cars, that may be sent direct to the 
retort houses, or to temporary coal storage pile at a rate as great 
as 250 tons per hour per crane. 

An electrically driven storage bridge 600 feet long (15) with a 
7 gross ton automatic bucket transfers at the rate of 300 tons per 
hour, the coal from the temporary storage to the storage yard, or the 
unloading crane first mentioned may reclaim the coal from the 
temporary storage and place it in 50 ton cars for its journey to the 





Fig. ile. 


retort house. The storage bridge at appropriate times transfers coal 
in storage to the same 50 ton cars, or when conveniently and hap- 
pily located, may deliver the coal directly to the track hoppers in 
front of the retort house. 

Ordinarily a 40 ton steam locomotive places two 50 ton cars con- 
taining different grades of coal side by side on two parallel sur- 
face railroad tracks at one end of the retort house; beneath the 
tracks is a hopper into which the cars are unloaded simultaneously 
at varying speeds. Usually one car contains a very sulphurous while 
the other contains a less sulphurous coal, and thus a uniform mix- 
ture of coals is obtained. The track hopper contains a chain scraper 
conveyor which moves the coal at the rate of 125 gross tons per hour 


ae 


MANUFACTURE AND DISTRIBUTION OF GAS 341 


AA 


Wand V7 Nae 
PRR M, 





Fig. 12. 





Fig. 13a. 


342 ILLUMINATING ENGINEERING 


up an inclined plane dropping it at the end into coal crushers, which 
discharge the coal uniformly crushed into vertical elevators which | 
raise the coal to the roof of the retort house where the coal falls on 
to longitudinal conveyors, which in turn distribute the coal into 
longitudinal coal bins in the inclined retort house, and, with the 
aid of cross conveyors store it in large bins convenient for charging 
machines in the case of the horizontal retort house. 

In the inclined house (16) the coal drops by gravity into measur- 
ing hoppers which are manipulated by hand and the coal thus 
directed to its final resting place by gravity into a “D” retort 





Higs 13b: 


normally 16” x 26” x 20 feet long. In the horizontal house the 
coal drops into a charging machine electrically controlled (17) 
which is run on rails opposite to and below the level of all retort 
lids; this machine measures all charges and charges the retorts by 
means of large scoops driven into the gas retort, on releasing its 
charge of coal uniformly distributed in the retort the scoop is 
withdrawn. | 

While the charge remains in the retort which is heated on its 
exterior by the combustion of hot coke drawn from a previous 
charge, gas is being continually driven off through a seven inch as- 
cension pipe as will be presently outlined. After a distillation period 
that may vary from 4 to even 8 or 9 hours, the mouthpieces are 


343 


MANUFACTURE AND DISTRIBUTION OF GAS 


‘SPT “OIA 








344 ILLUMINATING ENGINEERING 


¥ 4 


mo ge hE hater 


PEIN TAO > % > ere nine 


ae a on one 
nee, , 5s Scan 





Fig, bos 





345 


F GAS 


O 


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i 


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— 
ae 


Higs. 56 


5 
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ate 





gee h ca RE eR 





ATING ENGINEERING 


MIN 


Lung 


346 





ICE SR. 





ua. 19. 


MANUFACTURE AND DISTRIBUTION OF GAS 347 





Hig Zo. 


| ae | ees 
Sena eames, 
\ » “wer 





Fig. 21. 


14 


348 ILLUMINATING ENGINEERING 





Fig. 22. 





349 


MANUFACTURE AND DISTRIBUTION OF GAS 


‘$6 “OI 





IN inate 


14* 


350 ILLUMINATING ENGINEERING 


slacked off, fired and opened, and the residue of the coal being 
deprived of its volatile matter and now called coke, is either, in the 
case of an inclined retort, allowed, by gravity, to fall into a con- 
veyor in front, or in the case of a horizontal retort, is pushed 
by an electrically controlled machine (18) somewhat similar in ap- 
pearance to the charger, on to an electrically driven longitudmal 
hot coke conveyor running below the mouths of all retorts. Some 


of this hot coke may be deflected into the bench furnaces for heat- 





Bigs oa: 


ing the retorts externally, but the larger proportion proceeds along 
the house, meeting sprays of quenching water in its travel, and 
drops at the end of its journey into transfer bucket conveyors 
which convey it horizontally and vertically into a coke storage bin. 
(19) By gravity, coke is delivered from the storage bin (20) to 
30 ton cars on a grade railroad and by means of 40 ton locomotives 
delivered over a track hopper (21) at the wharf. The coke falls 
to an electrically driven belt which delivers the coke (22) at a 
speed of 200 net tons per hour to a barge alongside. Up to this 
time all material has been handled by electrically driven mechan- 


MANUFACTURE AND DISTRIBUTION OF GAS 351 


ism, except in the case of the steam locomotive and the hand 
manipulated measuring hoppers in the inclined retort house. 

The barge is towed to the coke distributing points and by means 
of a belt conveyor within the barge located just over the keel, of a 
bucket elevator (23), and athwartship or cross belt conveyor in the 
bow (all driven by a single kerosene internal combustion engine) 
is delivered at the rate of 60 tons an hour on to an electrically 
driven inclined belt conveyor located on the wharf which (24) de- 
posits the coke on a coke platform. From the platform it is deliy- 
ered by gravity (25) over coke screens to teams or motor trucks 
which in turn deliver it into the sidewalk chutes of commercial 
buildings, apartment house steam plants or other users of coke. 
Only here, beyond the jurisdiction of the manufacturer of gas, is 
any hand labor applied since the coal left the pick of the miners in 
the mine from which it came. An exception to this statement 
would exist where coke has to be carried in baskets from the team 
on the street to the storage bins of the user. 

A convenient auxiliary to Retort Coal Gas manufacture is a Car- 
buretted Water Gas Plant. The fuel used to make the water gas is 
coke, and this should be delivered hot direct from the coal gas 
retorts but when necessary quenched coke is taken from the coke 
storage bins for use in the water gas generators in adjoining gen- 
erator house. Detail explanation is omitted for want of time, the 
explanation of the manufacture of carburetted Water Gas hereto- 
fore made El and 2 being deemed sufficient. 

As previously pointed out 1000 Ibs. in coke from each 2240 
lbs. of bituminous gas coal received at the coal wharf must be 
disposed of as a by-product in Retort Coal Gas manufacture; the 
great value of this primary by-product must be at once apparent. 

While the coal lay in its hot bed in the retort the 36% volatile 
matter was seeking an outlet (26) via the ascension pipe before 
spoken of. ‘The length of time the charge remains under distilla- 
tion depends upon the degree and uniformity of heat, the character 
of the coal and the distribution of the charge in the retort, as well 
as the conductive qualities of the retort, for all have their influence 
on the resulting products. The heating of a charge of coal distills 
the volatile matter and causes a gas pressure within the retort; it 
is not desirable to have too great a pressure accumulate because of 
loss of gas through the porous sides of the clay retorts; the gas 
outlet of a retort or ascension pipe terminates in a metal chamber, 


352 ILLUMINATING ENGINEERING 








Fiq. 27. 


MANUFACTURE AND DISTRIBUTION oF Gas 353 


called a hydraulic main located above the retort bench; the ascen- 
sion pipes are sealed in water originally but later by accumulations 
of hot tar and ammoniacal liquor, products formed from a portion 
of the 36% volatile matter in the coal that become liquid at the 
temperature of the gas on passing the water seal. To prevent ex- 
cessive pressure within the retorts a gas pump called an exhauster 





Fie. 28. 


is installed in a house (27) beyond the retort house and the ex-. 
hauster is run at a variable speed, by the aid of automatic governors, 
so that all the gas as driven off in the retorts is at once drawn away 
from the hydraulic main undey a partial vacuum sufficient to over- 
come the water seal in the hydraulic main and to prevent but a very 
slight pressure in the retort. 
Forty years ago cast iron retorts were in use in cecal gas benches 


nates as 


g 


i 
a 


sneer 


Hp 


j 


Cea ee 
% Silty 





MANUFACTURE AND DISTRIBUTION 


or GAS 





399 


Higa 


356 ILLUMINATING ENGINEERING 


and then no exhauster was thought necessary as gas could not 
penetrate in excessive quantities the cast iron retorts. In the 
Pintsch Oil Gas process cast iron retorts may still be used, but so 
far as I am aware, no coal gas works to-day employs their use. 
Frequently gas in traveling from the retort house to the exhauster 
house is cooled by atmospheric influences either in the pipes lead- 
ing to the exhauster house or by specially designed apparatus. The 





Fig. 32. 


gas temperature at the inlet of the exhausters closely approximates 
120° Fahrenheit. 

On passing through the exhauster outlet the gas immediately is 
under pressure, for the exhauster is then forcing the gas to the 
storage holder against the storage holder pressure, which varies, 
depending upon its height, as will be explained later; additional 
pressure is produced by backpressure in overcoming the resistance 
of the gas travel through apparatus on the way to the holder as 


MANUFACTURE AND DISTRIBUTION OF Gas 357 


well as by the skin friction of the main gas pipes of the system. 
Should the exhausters all stop simultaneously a safety gas blow out 
governor would allow the gas egress to the open air above the Ex- 
hauster house roof, thus preventing excessive accumulation of gas 
pressure between the exhausters and the retorts. 

Raw gas from the exhausters passes first through (28) mechan- 
ical tar extractors having plates with small openings that break up 
the gas volume in small streams and by friction disengages tar 
from these gas streams; the raw gas next passes through horizontal 
rotary scrubbers (29) where hydrocarbon liquids extract the naph- 
thalene in the gas. It is now believed that these washers should 
be placed next in order to the condensers later spoken of; from 
the naphthalene scrubbers the gas passes through a liquid solution 
of sulphate of iron in the (30) cyanogen washers which deprive 
the gas of any cyanogen contained. The lquid on saturation is 
sent to settling tanks, then to filter presses which form in a pressed 
cake a raw product called cyanogen sludge (31) which is shipped 
to the chemical factories; residue liquor from the filter presses is 
put through drying processes and converted into dry sulphate of 
ammonia, which is bagged and placed on the market for sale. 
After this purifying process the gas passes through the cast iron 
tubes of surface condensers: (32) the tubes are surrounded by salt 
water, where available, and the water current is arranged so that 
the stream of warm water leaving the condenser meets the warm 
gas entering the apparatus. 

On leaving the condensers the gas passes through water in am- 
monia washers (33) quite similar in their design to the Cyanogen 
and Naphthalene Washers, and the gas having been freed of all tar, 
cyanogen and naphthalene, now surrenders its last trace of am- 
monia. 

The tar from the hydraulic mains, the main pipe connections, 
exhausters, tar extractors and condensers is led into underground 
tar wells from whence it is pumped into shipping tanks near 
the wharf, from whence the chemical contractors take it to make . 
the tar into pitch, dead oil, and various coal tar compounds. 

The ammonia from the ammonia washers is sent to underground 
ammonia tanks, and together with ammoniacal liquor recovered 
from the hydraulic mains and other connections and apparatus 
where tar is present, is all transferred to ammoniacal liquor tanks 
near the wharf, from which the chemical contractors remove it and 


ILLUMINATING ENGINEERING 


358 





Fig. 34. 


MANUFACTURE AND DISTRIBUTION OF GAS 359 


obtain therefrom anhydrous ammonia, sulphate of ammonia and 
other ammonia products. 

The gas leaving the ammonia scrubbers next passes into the puri- 
fying boxes now usually a dry process of purification. Boxes 
(34) 40 feet square and 8 feet deep are uniformly spread with 
oxide of iron, usually deposited on white pine shavings supple- 
mented by iron borings. The layers vary in thickness in practice, 
being in some cases upwards of 42 inches thick. The gas is here 
deprived of sulphur which is in the form of sulphuretted hydrogen. 
Some fixed forms of sulphur are undoubtedly taken up from time 
to time in the journey of the gas from the hydraulic main to the 
outlet of the purifying house and unless a very sulphurous coal must 
be used, no hme purification is found necessary to meet a 20 grain 
legal provision. Where it is found necessary to use the latter it is not 
an extravagant statement to make that the increased cost of purifi- 
cation (more particularly in the case of Coke Oven Gas) may 
be ten times the cost of ordinary oxide purification. It has 
been found that in order to eliminate fixed sulphur compounds 
that all carbonic acid must first be removed from gas in one set of 
boxes then lime fouled from sulphuretted hydrogen will attack the 
fixed sulphur compounds in a second set of boxes, and finally a third 
set of oxide boxes must be used to remove any sulphuretted hydro- 
gen that may be present. These three independent processes must 
be carried on in the purifying house, where but one is required 
when using fairly low sulphur coal. One of the three processes (the 
second in order) is exceedingly disagreeable to the employees of 
the gas company and to the neighbors as well. So little value is 
now attached to the requirements for fixed sulphur compounds that 
England, having passed through the regulating by law stage, now 
no longer demands any specified freedom of fixed sulphur in 
gas; New York only very lately passed laws respecting sulphur, 
merely imitating the laws of other places without reference to any 
necessity. Massachusetts in this respect is also moderating its 
position as regards sulphur in illuminating gas. 

Having passed the purifying house the gas now goes through 
16 foot station meters, (35a-35b) in which the measuring drums | 
run in water; gas measurements are made as near 60° Fahrenheit 
as atmospheric conditions permit but the measurements are cor- 
rected to 60° Fahrenheit and 30 inches barometer in any event. 
Here I might state that unaccounted for gas, not leakage as fre- 


ILLUMINATING ENGINEERING 


360 





Fie. 35a. 


Baoan 





Fig. 35b. 


361 


CTURE AND DISTRIBUTION OF GAS 


A 


MANUF 








t 
f 


Gee ee 


a sspeye 


pales 











HIG sais 


362 ILLUMINATING ENGINEERING 


quently asserted, is ascertained by taking the sum of the readings 
of the station meters and subtracting therefrom the sum of the 
readings of all the consumers’ meters, the remainder is unaccounted 
for gas, which includes actual loss of gas by leakage, loss of volume 
represented by difference of temperature at which the meters in 


















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i P<P<P<pa MI 
AINZANTANIAAIAT 


PSs ASEM 



























SL RZNZMM 


ik AKA\A : 
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ase 













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5 ivi,i% 

















CROSS SECTION OF A GASHOLDER GFPOUNDED 


Fig. 38a. 


the cellars of houses measure their gas (which cannot be corrected) 
the slowness of the house meters, and condensation of hydrocarbons 
and aqueous vapor present in the gas itself. 

Passing from the station meter the commercial gas now goes into 
the storage holder. (36) The largest holder in use holds 15,000,000 
cubic feet; it is 300 feet in diameter and 233 feet high, having 48” 
inlet and outlet pipes. The illustration shows an empty tank pre- 
pared to receive a second holder. 


MANUFACTURE AND DISTRIBUTION oF Gas 363 


G. Distribution of Gas from Storage Holder to City Distribution 
Holder 

In small plants the works’ holder distributes the gas direct to 

consumers while in large plants Exhausters (in this service some- 

times termed boosters or Gas Pushers) (37) pump the gas from the 


V PSXX] ROOT BOXXX] OKOKER 


NALS 


Ke: VJ CZ eK 


ia eas 


XXX he i DEX O PO KL OOOOOOR] 


ZA 
\NZISNZINZINZIMIV. 
(tows 


























q [XXXXXXX] KXXXKXXXKM [XK KKK KKK XK KKK OOO 





























x) 

















vad 








x0: 








H 
= 


I _ XK 





HOLDER PROPER 
JUST PEFORE ENGAGING AN ADDITIONAL ‘SEC TION. 


Fig. 38b. 


works’ storage holder into transfer mains, as large as 60 inches in 
diameter, and some times through tunnels under rivers into district - 
Distributing Gas Holders. 

It might be well to here call your attention to the method of 
obtaining the initial gas pressure used in gas distribution. It has 
been stated that the gas exhauster withdraws the gas from the 
retorts as rapidly as made and that after the gas passes through 
the exhauster it is under pressure due to the skin friction of pipes, 


364 ILLUMINATING ENGINEERING 


the resistance to the passage of gas through the apparatus described 
and the pressure of the gas storage holder due to its weight. In the 
case of the district holder, identical in all respects to the works’ 
storage holder, the motive power (steam, gas or electric) driving a 
gas exhauster fills the holder and when gas has been forced into 
the holder the holder itself maintains the initial gas pressure on 
the supply mains in the following manner. 


{J} 
as 
Dx 


| 
| 
\ 


| 
| 
| 




















FOLDER VE OLLI Nil Avg eaae 


Fig. 39: 


The gas holder is free to move vertically and when being filled 
is held in its vertical position by guide wheels rolling on guide 
framing supported and bolted to the tank wall. The top of the 
holder prevents the escape of gas upward, the cylindrical barrel 
or sides of the holder prevent any escape from the sides and the 
water in the holder tank prevents escape downward; the only man- 











MANUFACTURE AND DISTRIBUTION OF GAS 365 


ner that gas reaches the holder or escapes from it is by way of 
pipes passing through the water of the tank; gas flow is governed 
by valves on these pipes in the valve house. The gas is in fact sup- 
porting the gas holder and the weight and area of the gas holder 
determines the initial gas pressure obtainable. 

It would be quite impossible to have a water tank as deep as the 
gas holders in our large cities are in height, which would be neces- 
sary if the gas holders were not made telescopic. The photograph 
shows (a) a (38a) cross section of a gas holder; grounded, in such 


Guive FRAME. 










Gas INLET. 














DETAILED CROSS SECTION 


SHOWING THE HYDRAULIC SEAL BETWEEN TWo OUTER SECTIONS 


Fie. 40. 


position giving no pressure whatever; (b) (38b) the holder proper 
just engaging an additional telescopic section; (c) (39) the gas 
holder entirely inflated with the first section or holder proper and 
its 4 additional telescopic sections filled. In the case illustrated the 
holder is 190’ 10” in diameter, 230’ high and weighs 2,170,203 Ibs. - 
The holder proper or first section will give an initial gas pressure 
measured at the crown of the holder, of 4.9 inches of water (you 
remember that 27.68 inches of water is equal to 1 lb. pressure per 
square inch) the second section when added increases the pressure 
by 2.8 inches and gives a total of 7.7 inches—the third a total of 
10.4 inches—the fourth a total of 12.5 inches—and the holder 


366 ILLUMINATING ENGINEERING 


fully inflated a total of 15.0 inches (if the holder were filled with 
air the air pressure would be 16.3 inches, the difference 1.3 inches 
is equivalent to a weight of 173,200 lbs. When the holder is 
grounded the water level in the tank is level, when the holder 
proper or first section is raised the water outside the holder is 
4.9 inches out of level with the interior and when the holder is 
entirely inflated (40) the difference in level is proportionately in- 
creased. The inlet and outlet pipes should be higher in level than 
the tank walls and the tank overflow always above the level suffi- 
cient to permit the maximum alteration in water level without 
wasting water. While the water visible within the tank rises very 
perceptibly the original water level within the holder is but shghtly 
altered because of the great difference of the water areas involved. 
The gas cannot escape from the telescopic joints because of water 
seals between the sections. When it is necessary to ground a 
holder and cut it off from the pipe system by valves, changes in 
barometric pressure or temperature would make the crown of the 
holder collapse, unless an opening to the atmosphere were made. 

If the maximum initial gas pressure required. does not exceed 
the gas pressure given by the holder proper or first section, then 
all the gas can be sent out of the holder. If a higher pressure than 
this is demanded then the gas exhauster must be called upon to 
supply the deficiency, for the holder cannot always be kept fully 
inflated, or ats value would be lost. 

I have taken some time to describe the gas holder, but it is the 
one feature of the gas business that is the envy of our electric 
brothers. What royalty would not an electric company pay for 
an equivalent electric device that would at an equal annual cost 
store, without loss, the latent energy of all their boiler and engine 
plant run for 24 hours, always ready at any second for maximum 
or unusual demand, and needing but the turn of a wheel or the 
automatic adjustment of a switch to bring its latent energy into 
action. The gas industry could not be what it is without the 
gas holder and without the aid of the hydraulic seal, as both are 
essential to gas manufacture and distribution. 


H. Distribution Holders to- Services 
The gas in the District Distributing Holders passes into a Valve 
House (41) where valve men may control the initial pressure and 
hence the rate of gas flow through the various large street mains 


MANUFACTURE AND DISTRIBUTION OF GAS 367 


20” to 30” or more in diameter which are connected later with a 
multitude of smaller street mains ranging from 4” to 16” in diam- 
eter. In special cases Gas Pushers are used to force the gas through 
the large mains for long distances before the gas is permitted to 
find its way into the smaller mains. The gas pressure to a district 
is regulated either by a gas regulator or by a valve man adjusting an 
ordinary gate valve. The valve man regulates the gas flow by 
watching the pressure of the gas leaving the valve house but in 





Fiq. 41. 


some cases is assisted by the use of an ingenious electrical device 
which, on pressing a contact key, rings a bell whose strokes indi- 
cate the pressure in the mains a mile or more away; in such a 
case the valve man maintains a given pressure at that distant 
point quite independent of what pressure the gas is under on 
leaving the Valve House. The best the gas manager can do is 
to strive to furnish a given locality with uniform pressure— 
the exact amount, within reasonable limits, is not so important. 
It is impossible to give a uniform pressure of gas everywhere in 


15 





368 ILLUMINATING ENGINEERING 


a gas system, for in order to distribute gas at all, difference in 
pressure must be established before the gas will flow in the pipes. 
Water distribution requires a pressure obtained by the use of reser- 
voirs, stand pipes, or pumps in order to make it flow through a 
city system. The water pressure of a city system is not uniform 
any more than the gas pressure in a gas system. 

In suburban districts the Gas Pusher is used to send gas many 
miles through small mains under so called high pressure; in this 
case we speak of the Gas Pusher as a Gas Compressor (it is usually 
of the piston type). At appropriate points in the system a branch 
pipe supplies a district through a reducing valve (usually in dupli- 
cate) into the local district system; the gas pressure maintained 
in such a district is the pressure that the district gas holder would 
furnish in ordinary circumstances; in fact a gas holder is fre- . 
quently primarily supplied by a high pressure main in which case 
the high pressure main is really a transfer main similar to that 
heretofore spoken of, only the diameter is small and the gas pres- 
sure carried is greater. In some cases gas is supplied directly from 
a high pressure main to a house en route; in such a case a gas pres- 
sure reducing valve is placed in the house, but a safety seal is also 
provided, so that if the reducing valve mechanism fails to perform 
its work, excess gas pressure cannot come on the house meter or 
house fixtures, but the gas seeks a safe course to the atmosphere 
above the top of the house by means of an escape pipe. 

The pressure carried in gas pipes in the street is quite inde- 
pendent of the strength of the gas pipe itself with respect to burst- 
ing by internal pressure. Any gas pipe would stand several hun- 
dred times the pressure it is subjected to in the ordinary district 
distribution of gas. 

The danger of fracture of gas pipes comes entirely from the char- 
acter of the soil and the use of the streets by other public utilities— 
electrical conduits, sewers, water pipes, steam pipes, conduits for 
telephone and telegraph and the like. During the installation and 
repair of these public utilities and following these processes the 
conditions underground are indefinite and complex. When a fault 
does develop in a gas main, the gas manager must have at hand 
emergency forces for instant service and it is not wise to give these 
men the task of coping with this useful servant under too high 
pressure, less it become a dangerous foe. Opinions may differ 
on the subject of gas pressures appropriate to use in public streets, 


f 


MANUFACTURE AND DISTRIBUTION oF Gas 369 


and fixed opinions are sometimes modified by extended experience— 
so that on this subject we can fairly say that “ circumstances alter 
cases ”. 

It is often suggested that the proper method of distributing gas, 
water, electricity, telephone service, steam and provide for sewage 
should be by installing all the conduits furnishing these in sub-sur- 
face chambers called pipe galleries (42) constructed the length and 
breadth of a town or city. The failure of many an untried but 
promising process is due to not taking into consideration all natural 





Fig. 42. 


influences and forces. Nature never fails to supply them all though 
even thoughtful and experienced men sometimes forget to take 
them all into consideration. ; 

Time does not permit discussion of the pros and cons of pipe 
galleries, but the writer is opposed to their use, and believes that. 
the pipe gallery cure is worse than the pavement disturbing disease. 
Corporations using the public thoroughfares should be required to 
restore the street surface disturbed to as good a physical condition 
as they found it. Under present conditions the maintenance and 
repair of the conduits of public utilities is conducted with a mini- 
mum of danger to the public; before advocating pipe galleries the 
possibilities of wide spread disaster should be first overcome. 

15? 


370 ILLUMINATING ENGINEERING 


I. Distribution of Gas from Street Mains to Appliances for 
Burning Gas 

Usually gas mains are drilled and tapped with standard pipe 
thread and gas services, usually the smaller sizes of wrought iron 
or steel pipes, by the aid of proper fittings conduct the gas to within 
the foundation wall of the consumers’ premises. It is good prac- 
tice to install a gas service in a straight line from the street main 
with a constantly rising grade to the house, provide at the street 





Fig. 43. 


main for reasonable settlement of street main, service or the soil 
supporting them, and provide a small sized opening within the 
house for access to the interior of the service pipe. Should the 
service pipe be exposed to atmospheric influences, as in the case 
of areaways in the front of the building, special precautions are 
advisable where the winter temperature may be expected to be very 
low. Artificial gas has in suspension aqueous vapors and vapors of 
hydrocarbon, which may be liquefied at low temperature. It is 
advantageous to have these liquids flow back to the street mains 


wy 


MANUFACTURE AND DISTRIBUTION OF Gas weal 


and collect into drips, which are chambers left in the street mains 
below the lower level of the mains. All gas mains in the street 
are very carefully laid on ascending and descending grades with 
drips installed at the low points. The drips are pumped dry from 
the street surfaces (43) by the use of a pump and receiving tank 
usually drawn through the streets by horses. 

Any condensation of the hydrocarbon vapors deprive the gas of 
both its illuminating power and its heat unit value and is always 









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provided against so far as possible. The aqueous vapor in severely 
cold weather may be congealed and thereby gradually reduce the 
capacity of the service pipe below the requirements of service. 
Naphthalene, usually in the fall season, sometimes gives trouble 
by obstructing the service pipes in a gas system. When the service 
pipe passes through an open area way the size is often increased, 
and the pipe is sometimes covered in some way, more particularly 
where entering the walls; this mitigates and sometimes eradi- 


372 ILLUMINATING ENGINEERING 


cates trouble with stoppages. It is wise to provide ample sizes 
of pipes for services dependent upon the gas demand. Some 
services are installed as large as the smaller sizes of street mains; 1n 
such cases the service pipes are connected with the street main in the 
same manner in which street mains are.connected with each other 
(44) at intersections of streets, usually with lead or cement joints. 
Service pipes are frequently provided with cut-off valves located 
between the street main and the building supplied; when so placed 
they should be of a type not apt to be inoperative through very in- 
frequent use. The pipe leading from the street service to the gas 
meter is frequently spoken of as the inside or house service and its 
location and installation is dependent upon the needs of the service 
and the will of the architect or owner. 

The gas meter in reality is a motor operated by the gas pressure 
originating at the gas holder but operating only when any gas 
outlet beyond the meter is opened to the atmosphere. Meters with 
drums revolving in water have gone out of use because of the 
difficulty of keeping the water level intact and from freezing 
troubles. Dry meters, so called, are almost universally used. 

The essential parts of a typical dry meter are two main cham- 
bers, one on each side of a central gas-tight partition; each cham- 
ber is fitted with a hollow collapsible bellows or piston formed 
by a flat dise connected with the central partition by a gas tight 
cylindrical leather diaphragm, each piston being inflated and de- 
flated within its own chamber and in rythm with its twin in the 
adjoining chamber beyond the central gas tight partition. The 
space within each piston and the chamber surrounding it are filled 
with gas and are independently connected by valve passages to a 
slide valve, usually of the familiar “ D ” pattern met with in simple 
steam engine practice, operating in a gas tight chamber at the top of 
the meter. ‘The mechanism is so connected that in reality the 
meter is a double acting motor. The gas is measured by count- 
ing on a dial located in the front of the meter the volumes 
corresponding with the number of times the collapsible pistons are 
inflated and deflated. If a piston is deflated the disc is close to 
the central gas tight partition and as it expands it dispels its 
external volume in gas from the chamber surrounding it. When 
it is deflated the volume of gas within it is dispelled. Before 
installing a meter the volume dispelled by these pistons is care- 
fully calibrated by means (45) of a small gas holder, whose capacity 


MANUFACTURE AND DISTRIBUTION oF GaAs ata 


is known by comparing its volume with that of a cubic foot bottle 
the accuracy of which is certified originally by the Federal authori- 
ties in Washington. If on test the dial mechanism of a gas meter 
indicates a measurement not within one per cent of the true volume, 
an adjusting device within the meter provides for correction that 
will produce a final result within that accuracy. 





Fig. 45. 


By custom and sometimes by law a gas meter (when tested on 
complaint for inaccuracy) is said to be correct or accurate if it: 
measures within 2% of absolute accuracy, and it is a safe statement 
to make that the public buys no commodity, wet or dry, that so 
closely meets the requirements of absolute accuracy as in the pur- 
chase of gas. 

Meters in use are subject to derangement but, fortunately for 
the consumer, they are more apt to become slow (that is, the dial 


374 ILLUMINATING ENGINEERING 


does not indicate the total volume of gas passing through the 
meter) than fast. Gas managers who do not consistently maintain 
the accuracy of the consumers’ meters will find that their unac- 
counted for gas will become larger. The certificate of a public 
authority as to the accuracy of the meter set by a gas company is 
a satisfaction to the consumer, but no well managed gas company 
requires such inspection as a spur to the maintenance of their 
accuracy, for the interests of the stockholders, whether private or 
municipal has always demanded careful supervision of this im- 
portant department. Gas meters are read when possible on the 
same date of each succeeding month, and bills are thus rendered 
for similar periods, which furnishes a basis for careful comparison. 
All consumers should learn to read their meters themselves; by so 
doing unnecessary waste in their use of gas is stopped. In a com- 
munity where one apartment may be occupied by several tenants 
during a period of one year, or where the consumer cannot pay 
the necessary deposit required by gas companies, it has been 
found desirable to install prepayment gas meters. ‘These meters 
are the same as other meters so far as gas measurements are con- 
cerned. There is a mechanical contrivance added that permits 
only that quantity of gas to pass the meter that is in value equal 
to a coin that is passed into the coin box by the consumer. As the 
last of this quantity of gas is passing the meter the valve mechan- 
ism reduces the outlet area which reducing the size of the gas 
flame warns the consumer that an additional coin is required in 
the money box. The coin attachment added to the ordinary meter 
largely increases the cost of the gas meter, and the coin collector 
must be added to the staff of the gas company. ‘The readings and 
accounts of the prepayment meters must be kept in the same man- 
ner as the ordinary meters. Prepayment meters should only be 
installed in locations convenient for immediate access and where 
the consumer only has control. 

Beyond the gas meter some consumers install a small gas govern- 
or; gas governors have their place but in practice when injudiciously 
installed they are of little value. As stated before the gas man- 
ager endeavors to furnish uniform pressure in a given locality. 
Where this is impossible a gas governor may be profitably used 
by the consumer. For photometrical or other delicate scientific 
work where absolutely uniform pressure is imperative their use 
is necessary. Gas pressure increases as the height of a building 


MANUFACTURE AND DISTRIBUTION oF GAS OTD 


increases, due to the difference in specific gravity of gas when 
compared with air; in tall buildings a gas governor in the base- 
ment cannot serve uniform pressure throughout the whole build- 
ing. ‘The increase in the pressure of coal gas may be roughly 
stated for example as one inch in a difference in height of one 
hundred feet ; because of this fact it is customary to locate the gas 
holder at the lowest point in a district to be supplied. 

For lighting (46) the most common method of burning gas is 
by the use of the ordinary metal or lava tip from which the gas on 
issuing and igniting first heats the carbon particles therein to in- 
candescence before they are totally consumed. These burners are 
the batswing burner in which the gas issuing from a narrow slot 





Fig. 46. 


forms a thin sheet of flame; the fishtail in which two circular 
streams of gas coming in contact with each other spread out on 
ignition in a similar sheet of flame, and the argand burner where 
a series of small cylindrical jets issue from a multiplicity of open- 
ings arranged in a circle forming a cylinder of flame enclosed by 
a glass chimney, and air for combustion is supplied from the bottom 
of the burner. These burners may be expected to give from three 
to five candle power per cubic foot of gas burned depending upon 
the quality of the gas used. 

The gas mantle (47) burner is displacing all other methods 
of burning gas for illumination. In this burner a partial mixture 
of air and gas is effected before the gas issues from the burner, 
and together with the air present at the burner outlet immediately 
affects complete combustion of the gas in a blue flame cone, which 


376 ILLUMINATING ENGINEERING 


coming in contact with the gas mantle renders it incandescent. 
Such mantles are either upright or inverted as the case may be; 
the candle power per foot of gas is five times that of a flat flame 
burner, but the mantles must be of good quality and properly ad- 
justed and maintained to give these results. It is for this reason 
that many gas managers are endeavoring to maintain mantle burn- 
ers for as low a price as possible to cover the cost of this work. 
In nearly all other appliances using gas it is burned to a blue 
flame as in the case of the mantle burner, and it is this fact that 
is tending to make a candle power requirement for gas obsolete, 





Rig. 47: 


for where a blue flame is required only the heat units in the gas 
are of importance. A dual standard is illogical and imprac- 
ticable for it is a fact that candle power and heat units do not 
rise and fall in direct proportion even in the same kind of gas, 
and there is a wide difference in the heat units in different gases of 
the same candle power as measured by the flat flame or the argand 
burner. The kitchen gas range burners all use blue flame, many gas 
heating appliances likewise, though some radiator types of heaters 
use a flat flame burner. Gas logs usually use no primary air for 
the reason that some light is desirable to simulate a genuine wood 
fire. Gas to-day is used in many more ways for heating than for 
lighting, and it has been asserted that more gas is used to-day for 
heating and manufacturing than for lighting; it is a fact that the 
proportion of gas used in daylight hours is on the increase. 


MANUFACTURE AND DISTRIBUTION OF GAS B77 


K. Piping of Buildings, Etc. 

Before the era of high buildings, and the advent of a cheap and 
certain supply of electricity it never occurred to the architect to 
leave gas piping out of buildings, but it is not uncommon to-day— 
and the tenedency is acquiesced in by our electric brethren. The 
piping and wiring of a ten or twenty story building for water, toilet, 
sprinkler system, steam heat, refrigeration, electricity, telephone 
and gas is a work of no small proportions and a great expense. 
The natural impulse is to consider what can be dispensed with, 
and two methods of illumination being now seemingly unnecessary, 
gas pipes are being omitted where possible. In commercial build- 
ings main risers are installed for the use of gas emergency hall 
lighting and for manufacturing purposes and in large apartment 
buildings for cooking and heating and emergency hall lighting. 
Many thousand dollars are thus saved the builder and owner. The 
consumer is then cut off from a choice of central station illumi- 
nant in so far as gas and electricity is concerned, though strange to 
say oil lamps are still in use even where both gas and electricity are 
available, and candles are also used in great numbers. It is a 
mistake to leave gas piping out of buildings, but it is unnecessary 
to pipe the large buildings so thoroughly as would needs be if elec- 
tricity were not available. Buildings which have a private electric 
plant should always be piped for gas for lighting purposes, for it 
is frequently needed. 

The modern builder to save the last dollar of construction cost 
desires to save all the steel possible by having the dead floor weight 
as small as possible and hence cinders with and without concrete 
are used above tile or brick arches. The pipes are sometimes em- 
bedded in the cinders and concrete. In course of time the sulphur 
in the cinders attacks the iron of the pipes laid in the cinders, and 
cases are many where entire piping systems have had to be aban- 
doned because of the pipes becoming unserviceable. This deteriora- 
tion in pipes is not confined to those used to conduct gas. When 
this system is used all pipes should be exposed or protected from — 
possible corrosion. 

In a building in New York there has been adopted a method of 
making the gas lighting and gas heating interdependent. ‘The 
building is lighted by gas, its heat is utilized for the heating and 
when the weather is cold thermostats installed on each floor open 
valves on radiators which thereby diminishing by radiation the 


378 ILLUMINATING ENGINEERING 


steam pressure coming from a gas heating boiler in the basement 
causes a valve actuated by steam pressure to admit more gas 
to the gas burners furnishing heat to the heating surfaces of the 
boiler, thus supplying to the floor requiring it additional heat. 
Shutting off all light would still further increase the gas burned 
under the boiler, while increasing the gas light would automatically 
decrease the gas used under the boiler. This building is ventilated 
thoroughly with the exhaust fans used in the modern systems of 
building ventilation. The results are satisfactory at this writing. 
The gradual extended use of mantle lighting tends to a return of 
the former general practice of piping buildings for gas whether 
electricity is used or not. It is well that this is so. The writer 
believes that all buildings should be thoroughly piped for the use 
of gas. 
L. Appliances Used for Burning Gas 

This subject is rather beyond the limits of this paper and I will 
content myself with the enumeration of the uses of gas as follows: 

For Lighting—By flat flame, argand and gas mantle burners, 
and through the agency of the gas engine, electric lighting is 
available. 

For Household Use (48a) Cooking, heating, gas ironing, hot water 
heating and heat for many small appliances, such as coffee pots, 
chafing dishes, hot water kettles, curling irons, ete. 

For Commercial Uses Hot water, instantaneous and automatic 
(48b), hotel ranges, broilers, caldrons, engines, smelters, melting 
furnaces, singeing furnaces, china firing, smoke houses, biscuit 
baking, steam boilers for feather and hat manufacturers, as well 
as for heating, gas irons and mangles, washing machines and a 
multiplicity of other uses which are increasing daily. 

For commercial uses it is common both for hight and heat to 
have the gas under increased pressure, or the air supply under pres- 
sure, or both and in most appliances primary air is mixed with 
gas but not in sufficient amount to make a mixture that would be 
explosive as in the case of the gas engine. When increased pres- 
sure is used for lighting the efficiency of the gas mantle light in the 
use of gas is doubled. Any method that uses the minimum amount 
of air for complete combustion will give the maximum temperature 
and hence increased efficiency per unit of gas used. ‘The steam 
boiler is vastly different in its efficiency depending upon how many 
pounds of air is used per pound of fuel, and the same principle 
applies to the use of gas as fuel. 


379 


MANUFACTURE AND DISTRIBUTION OF GAS 





Fic. 48a. 





Fie. 48b. 


380 ILLUMINATING ENGINEERING 


M. Influences Governing the Selection of a Particular Type of 
Gas for Adoption 

First: The first consideration should be the laws existing or 
that reasonably may be expected to be enacted; if the public de- 
mand is for a high candle power, to be obtained by a flat. flame 
burner, carburetted water gas best fills the requirements but in a 
very small community acetylene gas is the choice if calcium carbide 
is readily obtainable. : 

Second: A second consideration is an adequate certain and 
cheap supply of raw materials. In connection with this your atten- 
tion is directed to a map issued by the U. S. Geological Survey, 
entitled “ Known productive Oil and Gas Fields of the United 
States in 1908 ” and a second, entitled “ Coal Fields of the United 
States ” probably also 1908. 

In the manufacture of carburetted water gas unless a hard fuel, 
either anthracite coal or oven coke, as well as oil is available then 
coal gas would have to form part of the gas plant if for no other 
reason than to supply retort coke for the water gas generators. 

If oil is available in great quantities at a very reasonable price 
as in California then the oil gas referred to under head D5 and 
5 may be chosen. 

If gas coal is very cheap then coal gas might be the choice, ex- 
cluding carburetted water gas or oil gas, provided the candle power 
provisions do not make it imperative to use these gases. 

Third: A third consideration is the variation in the demand 
which may compel the use of carburetted water gas, even where 
coal gas is the natural choice, for the ease of supplying sudden 
large demands, and the small standby cost for materials combined 
with the smalier capital cost per unit capacity may make this gas 
cheaper to use in part, even if its cost per unit made seems actually 
greater than the cost of the same unit of coal gas. The coal gas 
plants of large cities in all parts of the country, even in the bi- 
tuminous coal regions, are for this reason supplied with carburetted 
water gas plants. 

Fourth: ‘The capital charge may oftentimes determine choice. 
Where the difference in cost of production is in favor of coal gas 
remember that the capital cost of equivalent capacity of water gas 
is materially less than coal gas and when the productive cost and 
capital cost are combined the choice may compel the use of car- 
buretted water gas. 


MANUFACTURE AND DISTRIBUTION OF GAS 381 


Fifth: The land area available is of importance for the space 
occupied by a coke oven plant is much larger than a retort coal 
gas plant of equal capacity and a carburetted water gas plant very 
much smaller than either. Where a small site area is the governing 
feature then carburetted water gas may be the type of gas best 
adapted for the conditions. 

Sixth: When labor is scarce and remuneration high that process 
demanding least manual labor may be the type to be chosen. Car- 
buretted water gas frequently best fills this requirement. 





Fig. 49.—The First House Lighted by Gas in England. 


Seventh: Where the demand for metallurgical coke is great and 
imperative, the coke oven process will be installed in any event, 
in which case one of its by-products, gas, may be utilized for the 
supply of gas in its neighborhood. A carburetted water gas plant 
is in this case a useful auxihary, for in dull business seasons when | 
coke is a drug on the market, the coke ovens may necessarily be 
shut down and the gas demand may be obtained for the time being 
from the water gas plant. Be sure that the coke demand is reason- 


382 ILLUMINATING ENGINEERING 


ably certain before building a coke oven plant for furnishing a 
commercial gas supply, or financial disaster may result. 

Eighth: The only available site for the gas works may be in 
a district where the choice may be the plant producing the minimum 
amount of dust and dirt. Here acetylene gas or oil gas or car- 
buretted water gas may be the choice to be made. 

Ninth: In addition to the choice of the type of gas best adapted 
to the situation there may be here included a consideration of the 
system of distribution to be adopted. For suburban districts with 





Fig. 50.—The First House Lighted by Gas in Baltimore. 


villages quite widely separated, each too small to support a gas 
plant, a high pressure system may be installed and the manufac- 
turing plant located in the town best situated commercially for 
economical production costs. 

Tenth: Some states have seen fit to pass laws as to maximum 
price at which gas may be sold. In such cases it would be well 
to consider fully this rather unusual condition, for it might well be 
that while the community might be glad to have a supply of gas 
even at greater charge than the law permits, it might be many 


MANUFACTURE AND DISTRIBUTION OF GAS 383 


years before a plant would pay a return at the maximum per- 
missible price. 

Eleventh: <A word of warning with respect to a new plant; do 
not make the mistake of not looking many, many years into the 
future. Lay the plant out on paper for the future extensions that 
are in prospect, for if they are not to be expected, consider care- 
fully whether the plant should be built at all. It does not cost 
money to look ahead and design a plant showing what is to be done 
in the future—but it costs money to build a plant to-day and within 





Fic. 51.—The First House Lighted by Gas in New York City. 


ten years tear it down to build a second and then repeat the process. 
Depreciation by inadequacy is a cost which is present in the ma-- 
jority of undertakings in a growing country. Minimize this so 
far as possible. Do not actually build for business so far in the 
future that interest and depreciation exceeds the saving made by 
erecting the structures in a single operation. Depreciation by 
obsolescence cannot be foreseen; be careful to install tried up-to- 
date apparatus. 


ILLUMINATING ENGINEERING 


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386 ILLUMINATING ENGINEERING 


N. The Future of the Artificial Gas Business 


Judging by the past the gas business is destined to grow in the 
future as in the past. Gas stockholders received a severe fright 
when electric light was introduced,. but there is little to fear for 
the growth of the gas business because of electricity until someone 
invents an economical and successful process to manufacture elec- 
tricity direct from coal. The fact that gas continues to hold its 
own even where electricity is manufactured on a large scale with 
water power makes it unlikely that even such an invention will 
seriously retard the growth of the gas business. Gas for all pur- 
poses where heat units are essential is more economical and is more 
than holding its own with electricity up to date. It is quite clear 
that the candle power provisions for gas are bound to be eliminated 
from legal requirements if for no other reason than that the burner 
now specified in many statutes will sooner or later go out of com- 
mon use as the argand burner has disappeared. Should the supply 
of available oil be removed, as naphtha within twenty years, then 
high candle power requirements must go. ‘The cornerstone of the 
gas business of the future, as it was before the discovery of oil, is 
gas made from bituminous coal and that reason has influenced 
the selection of a coal gas plant primarily for illustration, but not 
only for that reason but because its auxiliary machinery and manu- 
facture is the more complex. 

While the use of coal gas may increase as pointed out heretofore 
it is necessary at all times, though more particularly in large 
cities, to have a water gas plant in combination with coal gas to 
meet sudden demands for large quantities of gas due to atmospheric 
variations from day to day, and further the use of gas has displaced 
in so many ways the use of gas coke that the utilization of this by- 
product of coal gas by its manufacture into water gas seems im- 
perative. 

At this writing the supply of oil shows no prospect of failing for 
gas making purposes and the use of carburetted water gas is in- 
creasing, but in a paper of this kind a reference to such a remote 
possibility, in view of the constantly increased use of the oil produc- 
tion for other purposes than gas making, seems not out of place. 


m 


VIII 
PHOTOMETRIC UNITS AND STANDARDS 


ae Epwarp B. Rosa 


CONTENTS 


I. PHOTOMETRIC UNITS AND NOMENCLATURE 


The luminous flux (F) flowing away from a light source falls upon and 
illuminates other bodies, the illumination (E) being the flux per 
unit of area. The flux per unit solid angle is the intensity (I). 
-F is measured in lumens, I in candles. The flux per unit of area 
from a surface is the radiation (H’). 

The luminous flux is the radiant power multiplied by the stimulus co- 
efficient, which is a function of wave length. 

The mean spherical intensity I, is the total flux F (in lumens) divided 
by 47; the intensity I in a particular direction is TAU to 
the rate of flux in that direction. _ 

By analogy with an electrically charged body, the total quantity of 
light Q ona body is the surface integral of the specific intensity e. 
Total flux F equals 7Q. 

Case of extended sources. Disk, plane, cylinder, sphere. Law of in- 

_ verse squares for case of sphere and circular disk. 

Reciprocal relations between radiating bodies. Luminous flux within 
an enclosure. Equations of definition of photometric magnitudes. 


II. PRIMARY -AND SECONDARY. PHOTOMETRIC STANDARDS 


A photometric standard is a standard of light flux, either its total flux 
or (more commonly) its rate of flux in a particular direction being 
taken. j 

The international candle is a unit and not a standard. 

The two kinds of primary standards employed in physical measure- 
ments; (1) those which are verified or reproduced from standard | 
specifications, and (2) those which are arbitrary and cannot be so 
reproduced. Examples of such standards. 

Flame standards are primary photometric standards of the first kind, 
although as used are often considered as of the second kind. In- 
candescent electric lamps are generally employed as secondary 
standards, but are sometimes. used as primary standards of the 
second kind. Other primary standards. 


16 


388 ILLUMINATING ENGINEERING 


The most important flame standards are the Harcourt pentane lamp 
and the Hefner amyl acetate lamp. Advantages and disadvantages 
of each discussed. Difficulties of both lie (1) partly in the lamp, 
(2) partly in the fuel, and (3) partly in the atmosphere in which 
combustion takes place. Each of these three questions discussed 
for each lamp. The pentane lamp gives more consistent results for 
a single lamp, but different lamps disagree more than is found for 
the Hefner. The important difference between a primary standard 
and a working standard. 

Preparation and calibration of electric lamps as photometric standards. 
Their performance as precision standards, primary or secondary. 
Method of measurement. Direction of improvement in primary 
standards. 


Introduction 


A discussion of photometric units and standards may be divided 
into two separate parts, the first including photometric units and 
nomenclature, and the second primary and secondary photometric 
standards. 

The development of the subject of photometric units and nomen- 
clature received a notable impulse through the paper of Professor 
Blondel, presented to the Geneva Congress of 1896. Since that 
time various modifications of the proposals then made have been 
put forward, but no authoritative action on the subject has ever 
been taken by any national or international body. ‘The nomen- 
clature as approved by the Geneva Congress has, in part, come into 
general use. There has, however, been a tendency to recognize as 
few separate photometric quantities as possible, and some of them 
have been employed rather loosely in more than one sense. This 
is partly at least due to a lack of clearness in the perception of 
the physical relations of the various photometric quantities. 


I. Untts aND NOMENCLATURE 
1. Case of Point Source 


We start with the idea of light as a luminous flux radiating or 
flowing away from the source, and illuminating bodies as it falls 
upon them. In the simple case of a symmetrical point source the 
flux is equal in all directions, and since the entire flux falls uni- 
formly, upon the interior surface of any concentric sphere, the 
quantity of the luminous flux.per unit of. area is inversely pro- 
portional to the square of the distance from the source, a law which 
has been verified by experiment. The quantity of the luminous 


PHOTOMETRIC UNITS AND STANDARDS 389 


flux per unit of area, or the flux density at the surface of the 
illuminated body, is by definition the specific wWlumination E. If 
we represent the total flux by F we have, therefore, 
F 
ihe i? (1) 
where r is the distance from the point source to the body il- 
luminated. 


} 


Representing es by a single letter I, we have 
Tv 


and 
F=47I (3) 





Fic. 1.—Hollow sphere of radius r, and surface 4rr’, with a Symmetrical 
point source at center, has a total flux F uniformly distributed over it. 


I is called the intensity of the source, and is equal to the flux 
per unit of solid angle. 

The wzlumination is equal to the intensity of the source divided — 
by the square of the distance (equation 2), and the total flux is 
47 times the intensity (3). 

The intensity I is measured in candles,* the flux F in lumens, 
and the distance r in centimeters. ‘Thus, from a point source of 
intensity I candles, there is a luminous flux 4rI lumens. 


* It is proposed to call the new value of the American candle, which 
is the same as the English candle and the French bougie decimale, and 
which is also used by several other countries, the international candle. 


390 ILLUMINATING ENGINEERING 


The flua density is the luminous flux per unit of area (normal 
to the flux in the case of a point source), or the total flux F over 
an area S§ divided by the area. If the flux density is variable, 5. 
will be a very small area, and F the flux over that small area. 
Thus a pencil of flux F, from a point source falls on a small area, 
S,, about the point P, and the surface density or illumination is 
Fy 


oe (Fig. 2.) 





Fia. 2. 


2. Definition of Intensity for Unsymmetrical Sources 


For a symmetrical point source the intensity I has been defined 
as the total flux F divided by 47. If the source is not symmetrical, 
but sends out a total luminous flux F unequally in different direc- 
tions, then the mean value of the intensity (F divided by 47) is 
called the mean spherical intensity Is. We thus define the mean 
spherical intensity with respect to the total flux; and, similarly, 
the mean hemispherical intensity is the ratio of the flux through 
a hemisphere to the solid angle 27, or the average flux per unit 
solid angle throughout a hemisphere. 

The intensity I in any particular direction is the quotient of 
the flux F, through a small solid angle » in that direction divided 
by the angle. Thus | 


Te ea, , » being a solid angle. 
@ 
Thus, if the pencil of the flux (Fig. 2) occupies an angle o, the 
intensity I of the source A in the direction of P is as ‘ Thus 
@ 


the intensity I is the angular density of the flux, as the illumina- 
tion E is the surface density. Thus the expression “ the intensity 
of a source in a particular direction” means the angular density: 


PHOTOMETRIC UNITS AND STANDARDS 391 


of the flux in that direction. Therefore, both E and I are flux 
ratios, lumens per unit area and lumens per unit solid angle, re- 


spectively. One lumen per square meter is the lux, and 1 lumen 


per unit solid angle * ( i of the total angle about a point) is a 


candle. 
If the source is not a point but a is a sphere of radius a, the 
flux 47I passes out from a radiant surface 47a?. Thus the flux 


density of radiation, or the specific radiation, is 


Fr 471 i | 
S ~ 4ra? a? pas: (4) 
Thus we may speak generally of the luminous flux at any point 
in space, and of the flux density of such radiation. If it falls on 
a material surface the incident flux density is the specific illumi- 
nation KH; as it comes from a luminous or other radiating or dif- 
fusing surface, the flux density is the specific radiation H’. Al- 
though E and EH’ are quantities of the same nature, it is con- 
venient thus to distinguish them, and for brevity we may often 
omit the adjective “ specific.” | 
The luminous flux density in space is analogous to electric dis- 
placement in electrostatics, which is represented graphically in 
direction and magnitude by lines of force, which start from posi- 
tive electricity and terminate upon negative electricity. .We think 
of an electric displacement as occurring in space between two elec- 
tric charges, but a surface density of electricity occurs only where 
there is a material conducting body on which the lines of force 





terminate. In the same way the terms luminous flux and flux 


density apply generally, both at the surface of the luminous and 
the illuminated bodies, and in the space between. The radiation 
is the flux density at the source of the flux, and the idlumination 
is the flux density or flux per unit area on the surface where Mie 
luminous flux is received. 


8. Distinction between Luminous Flux and Energy 
The total luminous flux F is not to be confused with the total 
energy flowing from a luminous body. Luminous flux, or light, 
as we ordinarily say, is the physical stimulus which applied to the 


* This is an angle subtended by 47 of a spherical surface, and in the 
case where the solid angle is a circular cone, its section through the 


apex is a plane angle of 65° 32’ 28”. 


392 ILLUMINATING ENGINEERING 


retina produces the sensation of light. It is equal to the radiant 
power multiplied by the stimulus coefficient. This stimulus co- 
efficient is different for every different wave frequency or wave 
length, and is, of course, zero for all frequencies outside of the 
visible spectrum. Herice, if W, is the power (expressed in watts) 
for unit of wave length of the spectrum, and K, is the stimulus co- 
efficient or lwminous efficiency whose value varies with the wave 
length 4, we have for the total power radiated from a body 

W=jW,da, (5) 
the integration being carried through the whole range of wave 
lengths, including non-luminous radiation. 

For the luminous flux, 
the integration being ditonahout the visible spectrum, K_ being 
zero elsewhere. 

As the values of K, throughout the spectrum are not sncuteiatys 
known, it is not possible to calculate F in general. But by meas- 
uring W in watts and F in lumens, we can determine the ratio 
of the luminous flux to the radiant power in any particular case. 
One may properly say that luminous flux is due to and is always 
associated with radiant power, but luminous flux and radiant power 
cannot, in general, be converted into one another like feet and 
inches; for, as stated above, the conversion factor, the stimulus 
coefficient or luminous efficiency, is not a constant like the ratio 
of feet to inches, but is variable, having a different value for every 
different wave length in the visible spectrum and falling to zero 
outside the visible spectrum. ‘“ Luminous energy” should, there- 
fore, not be used as synonymous with “luminous flux.” 


4. Unit Disk 


Concerning a body charged with electricity, we have the two 
ideas, (1) the electricity of density o and total quantity Q on the 
surface of the charged body, and (2) the flux of force throughout 
the surrounding space, there being 47Q lines of force for a quan- 
tity Q of electricity. We do not believe in the fluid theory of elec- 
tricity in the same way that Franklin did, but we nevertheless find 
the idea of a surface density of electricity very useful. In the 
corresponding case with light we may have similarly two distinct 
ideas, (1) a surface distribution of light over a luminous area 
of density or specific quantity b, and total quantity Q, and (2) a 


PHOTOMETRIC UNITS AND STANDARDS 393 


luminous flux filling the surrounding space and producing an il- 
lumination E on any body equal to the flux per unit of area. 

We have so far defined illumination and intensity in terms of 
the flux. Let us now obtain their values in terms of the quantity 
of light on the surface of the luminous source. 

The illumination from a very small source is inversely propor- 
tional to the square of the distance from the source, and directly 
proportional to the brightness of the source. Hence, for a luminous 
plane of unit area, we may write 

B= 2. (7) 
where b is the total quantity of light on the 
disk of unit area, which we define as the 
brightness, and the radiation to P, at a dis- 
tance r is normal (Fig. 3). For a point P, at 
an angle e from the normal, the illumination 
would be (approximately) proportional to S 
the cosine of the angle e; if the area of the 
disk is S we should have 

ayes bS oe ees (8) 
Q is the quantity of light on the small disk of 
area S, and is equal to bS (Fig. 3). 

The total flux * over the hemisphere illuminated by the disk 
is rQ. 

Thus the total luminous flux F from a small plane disk is r 
times the quantity of light Q on the disk. 





Fig. 3. 


* This is found by integrating the expression for E over the hem- 
isphere. Thus. 


27r* sin e cosede 





r= [finger sinede=Q | P 
0 r 
F—7rQ [sin’e]* TEs (9) 


In electrostatics there are 27Q lines of force on each side of a disk 
charged with Q units of electricity, or 47Q total. In the case of lu- 
minous flux, the flux is on one side only, and owing to the cosine factor 
the total is only one-half of what it would be otherwise. Thus, the 
total is only one-fourth of the flux in the electrical case. — 


394 ILLUMINATING ENGINEERING‘ 


The average illumination over the hemisphere of radius r is 
Pie lees 


Me whereas the maximum illumination En normal to the 
T 





disk is . Thus the mean is half the maximum. The intensity 
I has been defined as the angular rate of flux in any particular 
direction. It is, therefore, proportional to the illumination pro- 
duced in the given direction. Thus, in the case of the luminous 
disk we have h ? 

J,=maximum intensity, normal=Q, 

I,=mean hemispherical intensity = 2 : (10) 3 


I;=mean spherical intensity = os ; 


Thus F=71,=4r;,. ! (11) 
That is, the intensity is numerically equal to the total quantity of 
light on the small disk for all points on the normal. It decreases 
to zero as we pass 90° away from the normal, having a mean value 
of half the maximum for the whole hemisphere, and is on the 
average only one-fourth the maximum for the whole sphere. We 
may, therefore, say that the hemisphérical reduction factor for the 
disk is one-half, and the mean spherical reduction factor is one- 
fourth, the disk being supposed luminous on one side only. 
Since the total flux F from an area is 7Q, where Q is the quan- 
tity of light on the area, the flux from a unit of area is zb. This 
is the radiation EK’. Hence, in general, 
: Hi’ = ab. | (12) 
For a small sphere of radius a the total flux is 
F=K’ x surface. 
= rb X 4ra?=7Q 


Also 
F=4rlI. 
spa Qt 
mal hae om (13) 


That is, for a unit sphere * the intensity is one-fourth the quantity 
-of hight on the sphere. If the distribution of light over the sphere 
is not uniform, the mean spherical intensity is still one-fourth the 
total quantity of light on the sphere, as it is also for a disk. In 


e e e e s j 
* By unit sphere or unit disk, we mean a disk or sphere, the linear 
dimensions of which are negligible in comparison with the distance 
from source to receiver. 


PHOTOMETRIC UNITS AND STANDARDS 395 


other words, a sphere produces the same illumination at a given 
point as a disk of the same diameter and same brightness placed 
so that the radiation from the disk to the point is normal. 


5. Haxtended Sources 
(a) Circular Disk. Jet AOB represent a circular disk, lumi- 
nous on one side, of diameter AB, perpendicular to the paper. 
Kach element of the area sends out luminous flux toward the right 
in all directions (Fig. 4). Let us consider how much of this total 
flux falls upon a surface of unit area at P at a distance r perpen- 
dicular to the center of the disk. The intensity of the radiation 





Fig. 4. 


in any direction is assumed proportional to the cosine of the angle 
of emission, the radiation falling on the surface at P is also as- 
sumed proportional to the cosine of the angle of incidence. Hence, 
the flux falling on the area at P is less from the outer portions 
of the luminous disk AOB than from the center, not only because 
the distance is greater, but also because the two cosine factors are 
less than unity. Summing up the radiation from the whole disk, 
we find that the flux falling on unit area at P, which is the i-_ 
lumination, 1s 
imei G 

EES EEN, wtb 
where Q is the total quantity of light on the disk, and d is the 
distance to the edge of the disk.* 





* This is shown by integrating over the disk. See paper in Transac- 
tions Ill. Eng. Soc., June, 1910, p. 479. 


396 ILLUMINATING ENGINEERING 


We cannot define the intensity I of the disk in the same way 
we have for a point or a unit disk, for the radiation is not in a 
diverging pencil, as from a point. We can, however, define the 
equivalent intensity I, as the intensity of a small source at the 
center O, which would give the same illumination at P. If all 
the light on the disk were concentrated near O the illumination at 
P would be greater than that due to the disk. But, if a smaller 
quantity, Q,=Q ca were concentrated at O, the illumination at P 
would be the same. Hence, the equivalent intensity I, of the disk 
for the point P is =. But if the point P be moved nearer the 
disk, the equivalent intensity of the disk is less than this, for, 
will be smaller, and if the point P be further away I, will be 
greater. ‘Thus, the equivalent intensity of an extended luminous 
disk depends on the place at which the flux is being received, in- 
stead of being constant for all distances as it is for a point or a 
sphere. In general, the intensity I, or the angular density of the 
luminous flux, does not apply to extended sources. The quantity 
of light Q, however, has a definite meaning in every case. It is 
the surface integral of b, the brightness, and is not only a very 
useful quantity to employ in certain calculations, but tends to 
fix our ideas concerning luminous sources and facilitates exact 
expression. af 

In the case of a luminous cylinder of radius a and length 1, the 
quantity of light upon the conyex surface is Q=2zalb, b being the 
brightness. The horizontal illumination at a distance r, large in 
comparison with the length of the cylinder, is 

B= 
The total luminous flux F from the cylinder is 7Q, and, therefore, 
the mean spherical illumination on the inner surfaces of a con- 
centric sphere of radius r is 

ota Q area 

ae Ary? ta Ar? 

The spherical reduction factor f for the cylinder is the ratio E, 
divided by E,. Therefore, 


f= Aes eed = 0.7854= 78.5% approximately. 


Thus an incandescent lamp of one or more straight filaments 





PHOTOMETRIC UNITS AND STANDARDS 397 


should have a spherical reduction factor of 78.5 per cent. This 
is nearly the value for the tantalum and tungsten lamps, the base 
of the lamp cutting off some light, and so making it slightly less. 
A round disk, luminous on both surfaces, has a spherical reduction 
factor of 50 per cent. This, of course, assumes the cosine law as 
holding exactly. 

The Inghting 


The total luminous flux delivered in a given time, that is, the 
time integral of the flux, may be expressed in lumen-seconds or 
lumen-hours, according to circumstances. If this is called the 
lighting, and is represented by L, we have 

1D poses rl 
if F is the total flux in lumens and T is the time in seconds or in 
hours. The flash of a fire-fly may be expressed in lumen-seconds ; 
the total luminous radiation per gram of an illuminant, or the 
total lighting during the life of an incandescent lamp, may be 
expressed in lumen-hours. 

The following list of photometric quantities is substantially as 
recommended by the committee on nomenclature of the Ilumi- 
nating Engineering Society, and includes the quantities employed 
in the preceding discussion. 











TABLE [ 
Photometric magnitude Symbol Unit Equation of definition 
F 
1. Intensity of light I Candle i re 
it. Spe 
2. Luminous flux F Lumen F = Iw =n ES=7Q 
Lumens or Fr. 
3. Illumination E millilumens E= s = 
F 
4. Radiation i’ Ee 3 —rb=mE 
: Candles ted 
5. Brightness b ii | ae ey tona” 
6. Quantity . Q Candles Q=—bS 
7. Lighting L Lumen-hours L=FT 
I, b, Q are expressed in candles. F, E and EH’ are expressed in lumens: 
Pix = rb Besar G) 
KF; incident flux 
Fe —emergent flux 
m = coefficient of diffuse reflection or transmission 
(1— m) = coefficient of absorption. 
Yo mb 





*S, is a small plane area visible from the point for which the in- 
tensity I, is taken. 


398 ILLUMINATING ENGINEERING 


What is here called the brightness b has sometimes been called 
the specific or intrinsic intensity, and designated by i. But, if 1 
is the specific intensity, the integral of i ought to be the total in- 
tensity, and that is not true except for very small plane sources. 
For spheres, cylinders or extended sources of any shape, it is not 
true, and the term specific intensity is therefore unsatisfactory. 

On the other hand, the brightness b is defined as the quantity 
of light per unit of area, and the integral of b over the surface of 
a body, whether it be a self-luminous body of high temperature or 
a diffusely reflecting body of low temperature, gives the total quan- 
tity of light, Q, which multiphed by 7 gives the total luminous 
flux from the body. 


II. PRIMARY AND SECONDARY PHOTOMETRIC STANDARDS 


The fundamental quantity in photometry is the flux of light 
which produces illumination. We measure the flux from a given 
source by comparing it with that from a standard source. From 
a source of ight of mean spherical intensity I candles, a total flux 
of 47I lumens occurs. A standard source might be rated in terms 
of its total luminous flux in lumens, but owing to the fact that it 
is more convenient to compare accurately the angular rate of flux 
in a particular direction, or the mean horizontal rate of flux of 
two given sources, than to compare their total fluxes, it is better 
to rate photometric standards in terms of their intensity in a par- 
ticular direction in candles, or the mean horizontal intensity in 
candles than in terms of their total fluxes. Remembering that the 
intensity in a particular direction is proportional to the luminous 
flux in that direction, and is equal to the flux in lumens through 
a small solid angle w divided by w, we see that a standard source 
of any kind, though rated in candles, is really a standard of light 
flux, and 16 candles in a particular direction means a flux at the 
rate of 16 lumens per unit of solid angle in that direction. 

The international candle, as the common unit of intensity of 
England, France and America is generally and properly called, is 
a unit and not a standard. It will be continued by international 
co-operative effort, through frequent comparisons of the.material . 
standards maintained by the national laboratories of these coun- 
tries, but the particular standards that are employed by each coun- 
try in maintaining this unit have not been specified and need not 
always be the same. The comparisons are made by means of care- 


PHOTOMETRIC UNITS AND STANDARDS 399 


fully prepared carbon-filament lamps, and such lamps are chiefly 
_ employed in maintaining the unit constant. But flame standards 
may also be employed if they are found to be sufficiently reliable, 
and they can in any case be employed as checks upon the work 
done through the carbon-filament electric lamps, which are for 
the present, at least, more reliable. The latter are commonly 
called secondary standards, although in reality they are at present 
employed as primary standards. 


Two Kinds of Primary Standards 


The primary standards employed in physical measurements are 
of two kinds: (1) those which can be described in such terms that 
they can be accurately verified or reproduced from their specifica- 
tions, and (2) those which are more or less arbitrary, and which 
cannot be accurately reproduced except by copying other standards 
of the same kind. The international ohm is a standard of the 
first kind, as it is specified in terms of the resistance of a definite 
column of mercury at a certain temperature, and it can be repro- 
duced without reference to any other standard of resistance. The 
meter was originally intended to be such a standard, being defined 
in terms of the dimensions of the earth. But when it was found 
that the dimensions of the earth were different from what had 
been supposed, and that the meter would require a new definition, 
the reference to the earth was abandoned and the meter became a 
standard of the second kind, only to be reproduced by reference to 
other meter bars, copies of itself, of which there were a sufficient 
number in existence to make it possible to maintain the meter in- 
definitely in this way. More recently the meter has been expressed 
in terms of the wave length of light so exactly that it could be 
reproduced accurately if all length standards were lost. Hence, 
the meter has again become a primary standard of the first kind. 
However, meter bars are so permanent that in practice they are 
verified and reproduced by comparing with one another, without 
reference to the absolute specification in terms of the wave length 
of light. / 

The kilogram was intended to be a natural unit, so defined in 
terms of the unit of length and the density of water as to be a 
standard of the first kind. But, owing to the difficulty of deriving 
it in this way, it is more accurate, as well as more convenient, to 
regard it as a standard of the second kind, and to verify and re- 


4.00 ILLUMINATING ENGINEERING 


produce standards of mass by reference to well-made platinum 
standards without attempting to derive it according to its original _ 
definition. 

Thermometers are standards of the first kind, inasmuch as they 
are referred to the natural interval between the freezing and boil- 
ing points of water under standard conditions, and they can there- 
fore be verified or reproduced by referring to the formal speci- 
fications. 

The unit quantity of electricity, the international coulomb, is 
defined in terms of the quantity of silver it will deposit under 
standard conditions when passed through a solution of nitrate of 
silver. It is, therefore, a primary standard of the first kind. 


Primary Photometric Standards of the Furst Kind 


Primary photometric standards may be of the first kind or. of 
the second kind. Although primary standards of the first kind 
are to be preferred, other things being equal, obviously a reliable 
and convenient and permanent standard of the second kind is better 
than an unreliable, inconvenient and temporary standard of the 
first kind. Many primary photometric standards of the first kind 
have been proposed, and a considerable number have been used. 
‘The sperm candle is made to carefully stated specifications, and has 
‘been more widely used than any other photometric standard. But 
it is a very crude standard. The Carcel lamp in France, the Har- 
court pentane lamp in England, and the Hefner lamp in Germany 
are accepted as primary photometric standards of the first kind in 
the respective countries. They are made and used according to 
very elaborate specifications, but as the light is the result of the 
specified fuel burning in a specified lamp, surrounded by a speci- 
fied atmosphere, the standard is not merely the lamp, but the com- 
ination of lamp, fuel and atmosphere, the two latter of which are 
constantly changing. For use in ordinary gas photometry flame 
standards are convenient. But for precision photometry, in gen- 
eral, or for determining and maintaining a photometric unit, it is 
not unfair to say that the best of flame standards is not as con- 
venient or reliable as primary standards ought to be. 

The difficulties in the use of flame standards are, therefore, 
partly in the lamp, which is the more or less permanent part of the 
combination; partly in the fuel, which is often found not to con- 
form to the specifications, and in some cases is liable to change on 


PHOTOMETRIC UNITS AND STANDARDS 401 


standing even if it conforms originally to specifications, and partly 
to the atmosphere, which is constantly changing with respect to 
barometric pressure and aqueous vapor, while in and about the lamp 
it changes also with respect to carbon dioxide and oxygen content. 
All these variations affect the light as a flame standard, and make 
the errors of measurement many times greater than those made 
on carbon-filament lamps. By. making a long series of measure- 
ments, the accidental errors are largely eliminated, and a mean 
result may be obtained which is surprisingly good in view of all 
the difficulties. But there are constant sources of error that are 
not so eliminated, and perhaps the most perplexing are due to 
the lamp itself. For example, although Hefner lamps are made by 
different makers very carefully from the same specifications, there 
is a range of 2 per cent between the highest and lowest values of 
eight Hefner lamps belonging to the Bureau of Standards, four 
from one German maker and four from another. There are two 
different devices in use for observing the height of the flame, but 
all (or nearly all) the lamps conform to the specifications. If one 
requires only that his Hefner lamp be correct within 2 per cent, 
all these lamps are satisfactory. But as primary standards they 
ought not to differ so much independently of fuel and atmosphere. 
In the same way, standard Harcourt pentane lamps differ several 
per cent in candle-power, using the same fuel and operating them 
under the most favorable conditions. At the Bureau of Standards 
we have tested pentane lamps from two English makers and one 
American maker. The two Chance lamps tested have the highest 
candle-power, averaging about 9.9 international candles under 
standard atmospheric conditions, namely, 8 liters of water vapor 
per cubic meter of air, and standard barometric pressure. The 
Sugg lamps tested average less than 9.7 candles, about 2.5 per 
cent less than Chance lamps. American-made pentane lamps also 
average about 9.7 candles. 

The standard Harcourt pentane lamp was supposed originally to 
give 10 British parliamentary candles, and there was supposed to 
be no appreciable variation among different lamps. The National 
Physical Laboratory adopted a particular lamp of this kind as its 
primary standard. When the international candle was fixed by 
agreement between the national laboratories of England, France 
and America the Bureau of Standards made a change of 1.6 per 
cent in its photometric unit, in order to come into agreement with 


402 ITLUMINATING ENGINEERING 


England and France, and at the same time to bring the gas and 
electric industries of America to a common standard by bringing 
the new unit midway between the old unit of the Bureau, which 
- was used by the electrical industries, and the average value of the 
unit used in the gas industries. Theoretically, therefore, the 
standard pentane lamp should give 10 international candles. But 
it happens that the particular standard pentane lamp of the Na- 
tional Physical Laboratory apparently has a slightly higher value 
than the average, and the English maker of the lamp has been 
unable to furnish us a lamp giving the same candle-power. The 
Bureau placed an order for a lamp to agree with the standard of 
the National Physical Laboratory, as shown by direct compari- 
sons made at the National Physical Laboratory. After several at- 
tempts on the part of the maker a lamp was accepted having about 
1 per cent lower value. The Bureau has never tested a pentane 
lamp of any make having a value as high as the National Physical 
Laboratory standard. The values found range from 1 to 5 per 
cent less. Hence, it is evident that the pentane lamp as a primary 
standard cannot be a complete success until different makers fol- 
lowing the same specifications can produce lamps agreeing better 
in value, and until the lamps produced by any experienced maker 
agree better among themselves than they now do. At the Bureau 
of Standards we have made some progress in locating the source 
of the differences, and hope soon to see a great improvement in 
this respect. 

The second source of trouble with pentane lamps is the fuel. 
Pentane (C,H,,) is a very volatile hydrocarbon, distilled from 
gasoline. It is classed as explosive, and should be shipped in 
strong sealed cans and stored and handled with special precau- 
tions. It costs the Bureau of Standards $3.50 per gallon, and is 
consumed in considerable quantities. It is distilled between 25° 
and 40° C., and in summer in an open can evaporates rapidly at 
laboratory temperatures. The flame is ordinarily fed by the mix- 
ture of air and pentane which come over from the saturator. But 
in hot weather instead of air entering the inlet, pentane vaporizes 
so rapidly that it flows out through both outlet and inlet, the 
vapor escaping through the air inlet, passing out into the atmos- 
phere of the laboratory, and so causing the pentane to disappear 
at, perhaps, double the normal rate. Hence, pentane lamps, as 
ordinarily constructed, cannot be used satisfactorily in summer in 


PHOTOMETRIC UNITS AND STANDARDS 403 


southern latitudes. Slight modifications in the lamp can be made 
to overcome this difficulty. 

Moreover, as pentane is not a simple compound, but contains 
homologous compounds which are not completely separated even 
by repeated distillations, the density changes as evaporation pro- 
ceeds, and hence the reservoir must be emptied and refilled with 
fresh pentane from time to time, in order to keep the fuel within 
the specifications and the light of the flame sufficiently near to its 
normal value. 

The light of a pentane flame, like other gas flames, is very sensi- 
tive to impurities in the atmosphere and to drafts or air currents. 
There must be excellent ventilation of the room and plenty of 
pure air supplied to the flame, but not too much. The removal 
of the products of combustion and the screening of the lamp from 
air currents, as well as the regulation of the supply of pentane and 
the detailed manipulation of the lamp, all call for experience, pa- 
tience and skill in high degree, in order to get consistent and re- 
liable results from a pentane standard. 

Of course, the atmospheric humidity must be carefully deter- 
mined every time a set of measurements is made, and the barom- 
eter must be read in order-that humidity and pressure corrections 
may be made. ‘These corrections are considerable, the humidity 
correction, which is the larger of the two, sometimes amounting 
to 10 per cent. 

These remarks apply only to pentane lamps which are used for 
the purpose of relatively accurate measurements. As working- 
flame standards they may be used with fewer precautions, if ap- 
proximate results are sufficient. 

When a flame standard is employed for testing illuminating gas, 
the humidity and barometric corrections are not applied, as the gas 
flame is affected practically by the same amount, and the test is in- 
tended to demonstrate the quality of the gas and not the amount of 
light given by the given test burner at that particular time. In 
other words, 20-candle-power gas is not gas that always gives 20 
candle-power in a particular burner when consumed at a stated 
rate, but gas of standard light-giving properties, that is to say, 
it gives 20 candle-power when burned at a given rate in a particular 
burner in a standard atmosphere, which is a pure atmosphere con- 
taining 8 liters of water vapor per cubic meter and at normal 
barometric pressure. In winter, when the humidity averages less 


404 ILLUMINATING ENGINEERING 


than normal, the light will be greater than the average. In sum- 
mer, when the humidity averages greater than normal, the light 
will be less than the average, and may be 10 per cent less. Thus, 
flame standards are for this reason well adapted to serve as work- 
ing standards for testing the light-giving properties of gas and oil. 
But for primary standards, intended to maintain a photometric 
unit, they are not as well adapted as they would be if unaffected 
by the atmosphere. 

Hefner lamps have some important advantages over pentanes, 
and some marked disadvantages. Whereas the standard Harcourt 
pentane lamp is bulky, complicated in construction, relatively la- 
borious to manipulate, and expensive both in first cost and in fuel, 
the Hefner amylacetate lamp is small and very portable, simple in 
construction, easy to assemble and make ready for use, and less 
expensive in first cost and in fuel. The latter costs the Bureau 
$3.00 per pound, but so much less is employed that it costs less 
per hour than pentane at $3.50 per gallon. 

Its disadvantages in comparison with the pentane standard are 
(1) its small candle-power, (2) the redder color of its flame, (3) 
its more unsteady flame, and (4) the greater difficulty of main- 
taining the correct flame height. 

The Hefner flame has a horizontal intensity of 0.9 candles when 
the flame is 40 mm. high, as officially prescribed in Germany. We 
find at the Bureau that the flame burns about as steadily and is 
nearly as easy to manipulate when maintained at 45 mm., at which 
height it gives 1 international candle, or 0.1 candle more than at 
40mm. This change in a standard lamp is made by placing a 
ring 5mm. thick under the support of the sight which is used to 
regulate the flame height. To obtain a suitable illumination on the 
test screen of the photometer, a standard of 1 candle-power must 
be placed quite near, and errors due to slight variations in distance 
are much greater than for a 10-candle-power standard. In prac- 
tice, both a shorter distance and a weaker illumination are em- 
ployed with the Hefner standard. 

The color difference between standards, or between a standard 
and a light source, is necessarily a source of uncertainty, and with 
modern electric and gas lamps the demand is for whiter standards. | 
The Hefner is the reddest standard in use, and its color is one of 
its most serious objections. However, color screens are necessary 
to pass from one color to another, and the difference between the 


PHOTOMETRIC UNITS AND STANDARDS 405 


pentane color and the Hefner color is not enough to make this a 
deciding consideration, as between the two lamps. The voltage 
on a carbon-filament lamp necessary to give a color match with 
several different flame standards is as follows: 

To give 4 watts per candle =110 volts. 

To match the kerosene lamp =102-108 volts. 

To match the Carcel lamp =98 volts. 

To match the pentane lamp =91 volts. 

To match the Hefner lamp =86 volts. 

The flame of a Hefner lamp is very easily disturbed by air cur- 
rents, and the tip is in almost constant motion vertically and 
laterally, so that the flame must be screened very carefully, and 
then must be watched constantly by an assistant, and readings made 
only when it is at the right height and in correct position. The 
tip is only slightly luminous, and yet the height must be main- 
tained constant to a fraction of a millimeter. Different observers 
may differ sensibly in their judgment as to when it is right, al- 
though this source of error is smaller than would be supposed. 

The amylacetate is so volatile that the top of the wick is below 
the top of the wick tube. As the room temperature rises, the 
wick must be lowered to keep the height of flame constant, and 
this makes the flame more unsteady. At summer temperature, 
‘such as 25° to 30° C., the flame is much more unsteady than at 
15° to 20°C. In this respect (less satisfactory operation in hot 
weather) both the pentane and amylacetate lamps and candles are 
inferior to kerosene-oil lamps. 

Because the Carcel lamp is so little used in this country, or any- 
where outside of France, and because our limited experience at 
the Bureau has shown it to be unsatisfactory, nothing will be said 
of it as a standard. 

Candles have been of enormous service in practical gas photom- 
etry, but they cannot be seriously considered at the present day 
as standards. Kerosene-oil lamps are much more convenient and 
reliable, and we hope in the near future to publish experiments- 
made at the Bureau showing that as secondary standards for prac- 
tical photometry they may be used with excellent results. 

To sum up in a few words, it may be said that as primary photo- 
metric standards of the first kind, there are the pentane and Hefner 
lamps about equally entitled to consideration, each possessed of 
important merits, but also of serious hmitations and defects. A 


406 ILLUMINATING ENGINEERING 


given pentane lamp is probably more consistent with itself than an 
average Hefner, but different pentane lamps differ more than Het- 
ner’s do. No other standard of the first kind equals them in con- 
stancy and reproducibility, and no other is used where accurate 
results are attempted. 

The radiation from incandescent platinum at its melting point 
was long ago proposed by Violle as a primary photometric unit of 
the first kind. But, although enormous progress has been made in 
obtaining and maintaining and measuring high temperatures, and 
several serious attempts have been made to make Violle’s proposal 
practicable, nobody has ever succeeded in doing as well with it 
as can be done with flame standards. Drs. Waidner and Burgess, 
of the Bureau of Standards, have made an interesting proposal, 
namely, to employ the radiation from a black body at a particular 
temperature, for example, at the melting point of platinum, but 
they have not as yet attempted to realize it in practice. Dr. Stein- 
metz has recently also made a new proposal for a primary photo- 
metric standard of the first kind, but the realization of this pro- 
posal to the extent of obtaining a standard of precision seems very 
difficult, and so far,as I know has not been attempted. 


Photometric Standards of the Second Kind 


The most successful photometric standards of the second kind 
are carbon-filament incandescent lamps, which have been employed 
for many years as convenient working standards, and in recent 
years have been employed in making careful comparisons of the 
photometric standards of different countries. ‘Their use is so im- 
portant and their operation under the best conditions is so admir- 
able that I wish to present briefly the method of their preparation 
and use and records of their performance. Such lamps cannot, 
of course, be made accurately to specifications, but if they are 
sufficiently permanent they may be employed to maintain the unit 
of light for an indefinite period. Probably nothing is more per- 
manent than pure carbon, sealed in a vacuum and kept at ordinary 
(room) temperatures. Hence, if carbon-filament lamps can be 
prepared which will not change appreciably when burned, say 100 
hours, under working conditions, there is reason to believe that 
they will remain constant for a long period of years (barring acci- 
dents) and that a group of such lamps will afford a means of 
maintaining the unit of ight constant for a long time. How long 


PHOTOMETRIC UNITS AND STANDARDS 407 


and how accurately can, of course, only be determined by ex- 
perience. . 

Carbon-filament incandescent lamps are usually operated as 
standards at a constant voltage, the current being measured as a 
check. Sometimes they have been measured at constant current, 
the voltage being varied slightly, if necessary. If the lamps have 
constant resistance, of course these two methods would amount to 
the same thing. But, as carbon-filament lamps do not have con- 
stant resistance, but generally show a decreasing resistance, fol- 
lowed after a longer or shorter period by an increasing resistance, 
it becomes a matter of prime importance whether the best per- 
formance can be secured by operating lamps regularly during their 
useful life as standards at constant voltage, or at constant current, 
or whether still better results can be obtained by operating them 
at constant watts. Obviously, if the radiation from the surface 
of the filament is unchanged, and the bulb does not blacken or 
change its absorption, the most constant candle-power will be se- 
cured by operating the lamps at constant watts; a constant rate of 
energy supply and a constant conversion factor giving a constant 
flux of light. But, whether the radiation from the filament and 
the absorption in the bulb will be constant at constant watts could 
only be determined by experiment. 

At the Bureau of Standards we have investigated this question 
very carefully, and to obtain the highest possible precision have 
made use of a double-precision photometer, with special recording 
cylinders, having two observers measure the same lamp simul- 
taneously, and a third observer measuring the current and voltage 
of the lamp at once by means of two standard potentiometers. A 
single determination consists of the mean of a large number of 
readings, each recorded without the observer taking his eye away 
from the photometer, and as the observer does not know any of his 
readings until they are all completed, he reads without prejudice. 
By this means each observer is a check upon the other, twice as 
many determinations can be made in a given time as by a single - 
photometer, and by the use of Dr. Middlekauff’s direct-reading scale 
all calculations of candle-power are eliminated, the value of each 
determination in terms of the mean of the group of standards em- 
ployed being read off directly from the record sheet. 

In this way it has been found that with lamps in which no 
blackening occurs the best results are obtained by keeping the watts 


408 ILLUMINATING HNGINEERING 


constant instead of using them at constant voltage. Life curves 
have been made of a large number of standards, and each curve 
divided into the period of seasoning and the period of useful life as 
precision standards. If they are burned at constant volts, the season- 
ing is carried on until they reach constant resistance. This is a 
longer or shorter operation, depending on the temperature (or 
watts per candle) at which they are seasoned, but is not the same 
for different lamps. If, however, they are to be burned at con- 
stant watts, it is not necessary that the seasoning be continued to 
minimum resistance; when the filaments have nearly reached that 
condition they may be used with perfect satisfaction, and long after 
the resistance has reached its minimum and has increased appre- 
ciably the lamp is still a reliable standard, provided only that the 
watts have been kept constant, and, of course, provided that black- 
ening has not occurred. 

Blackening can be detected by the decrease in the light of the 
lamp before it can be seen on the glass. To reduce it to a minimum 
the lamps should be made and selected with great care, and the 
filaments should preferably be mounted in larger bulbs than is 
ordinarily done. Dr. Fleming, of London, many years ago advo- 
cated the use of large bulbs for incandescent-lamp standards, but 
as the quality of lamps improved it did not seem necessary to use 
them, and hence nearly all laboratories used the ordinary-sized 
16 candle-power lamps for standards of the best quality. We have 
found in our recent work at the Bureau of Standards, however, 
that lamps in larger bulbs give better results. | 

We have seasoned and carefully measured nearly 200 standards 
as above described, and selected the best for primary standards. 
A few of these have been burned for 200 hours after seasoning 
without the candle-power changing more than a few hundredths 
of a candle. Such lamps would serve as reference standards in a 
photometric laboratory for many years, perhaps for a century, with- 
out being burned as many hours as they have been burned in these 
special tests. There should be no depreciation while they are not 
burning, for what is more permanent than pure carbon, preserved 
in a vacuum at ordinary temperatures ? 

As to the precision of measurement of carbon-filament electric 
lamps, on such a precision photometer as described above, the mean 
error of the determination of candle-power on any lamp at one 
time is about 0.2 per cent, whereas the mean error of the average 


PHOTOMETRIC UNITS AND STANDARDS 409 


value of six lamps measured at one time is about 0.1 per cent. If 
a group of six lamps be measured by four different experienced ob- 
servers (as is done at the Bureau in work of the highest precision) 
the mean of the four will be still less in error. These figures are 
the results of a large number of experiments with rotating stand- 
ards, of the same color, and stationary standards may be measured 
with substantially the same accuracy. 

With such precision of measurement and a life performance of 
standards such as described above, it would seem as though the 
unit of candle-power not only of a commercial laboratory, but also 
of a national standardizing laboratory, or even of a group of na- 
tional standardizing laboratories, could be maintained for a long 
period of years by carbon-filament incandescent lamps more con- 
stant than has been possible heretofore with flame standards or 
any other form of primary standard as yet realized. 

However, there are possibilities of improvement in flame stand- 
ards, and, of course, possibilities of some new primary standard 
appearing which shall surpass any flame standard as yet proposed. 
What I wish to emphasize is, not by any means that incandescent 
lamps are the final standards or that they are satisfactory as pri- 
mary standards, but that they really are, as now used, primary 
standards, and that by their use a photometric unit can be main- 
tained so well that until the difficulties of heterochrome photome- 
try are overcome, and until the demands for precision in prac- 
tical photometry are considerably increased, we need not fear that 
the international candle will drift far enough from its present 
value to be serious. The progress that has been made in photo- 
metrical measurements in the 14 years since the Geneva Congress 
is gratifying. Then it was believed that the Hefner unit and the 
bougie decimale were practically equivalent. The uncertainty in 
the relative values of the standards of different countries amounted 
to several per cent. Now the corresponding uncertainty is not or 
need not be more than a few tenths of 1 per cent, so long as 
standards of a single color are employed. It remains to accomplish. 
as much for standards of a whiter color, and to fix the ratios 
in passing from one color to another. 


410 ILLUMINATING ENGINEERING 


REFERENCES ON UNITS AND NOMENCLATURE 


Blondel, A., Lumiére Elect. 53, pp. 7-15, 1894. 

Blondel, A., L’Eclair. Elect. 8, pp. 341-365, 1896. 

Broca, A., L’Helair. Blect. 6, pp. 148-157, 1896. 

Hefner-Alteneck, F. v., Elektrotech. Zeitschrift, 17, pp. 754-6, 1896. 

Kapp, G., Elektrotech. Zeitschrift, 17, pp. 531-4, 1896. 

Weber, L., Elektrotech. Zeitschrift, 18, pp. 91-94, 1897. 

V rband Deutscher Elektrotechniker, Elektrotech. Zeitschrift, 18, p. 
474, 1897. 

Millar, P. S., Elect. Rev. (New York), 51, pp. 426-8, 1907. 

Hering, C., Trans. Ill. Eng. Soc. 3, pp. 645-678, 1908. 

Rosa, E. B., Bulletin Bureau of Standards, 6, pp. 543-572, 1910. 


REFERENCES ON PHOTOMETRIC STANDARDS 


Hefner-Alteneck, F. v., Vorschlag Zur Beschaffung einer konstanten 
Lichteinheit, Hlektrotechnische Zeitschrift, 5, pp. 20-24, 1884. 

Phys.-Tech. Reichsanstalt, Die Beglaubigung der Hefnerlampe, Zeitschrift 
f. Instrumentenkunde, 13, pp. 257-267, 1893. 

Liebenthal, E., Ueber die Abhangigkeit der Hefnerlampe und der Pentan- 
lampe von der Beschaffenheit der umgebenden Luft. Zeitschrift f. 
Instrumentenkunde, 15, pp. 157-171, 1895. 

Vernon Harcourt, A. G., On a 10-Candle Lamp to be used as a Standard 
of Light, British Assoc. Report 1898, pp. 845-846. 

Fleming, J. A., The Photometry of Electric Lamps, Jour. Inst. of Hlect. 
Eng. 32, pp. 119-216, 1902-3. 

Paterson, C. C., Some investigations on the 10 c. p. Harcourt Pentane 
Lamp, Electrician (London), 53, pp. 751-752, 1904. 

Paterson, C. C., Investigations on Light Standards, etc., Jour. Inst. of 
Elect. Eng. 38, pp. 271-308, 1906-7; National Phys. Lab., Coll. Re- 
searches, 3, pp. 49-65, 1908. 

Dow, J. S., The Sources of Error in the Harcourt 10 c. p. Pentane Stand- 
ard, Elect. Rev. (London), 59, pp. 491-3, 1906. 

Glazebrook, R. T., The Photometric Standard of the National Physical 
Laboratory, British Assoc. Report, 1908, p. 623; Electrician (Lon- 
don), 61, pp. 922-8, 1908. 

Report of Committee on Taking Candle-Power of Gas, Proc. Amer. Gas 
Institute, 2, pp. 454-509, 1907. 

Bond, C. O., Working Standards of Light and Their Use in the Pho- 
tometry of Gas, Jour. Franklin Inst. 165, pp. 189-209, 1908. 

Rosa & Crittenden, Report of Progress on Flame Standards, Trans. III. 
Eng. Soc. 5, pp. 753-778, 1910. 


| IX 
THE MEASUREMENT OF LIGHT 


By Ciayron H. SHARP 


CONTENTS 


Photometry. 
Definition and scope. 
Quantities to be measured. 
Measurements. 
Are relative to a standard. 
Made by zero method, using the eye as instrument for determining 
equality. | 
Difficulty due to color difference. 
Apparatus, general. 
1. Sight-box, photometer head, or, for short, photometer, for pro- 
ducing contiguous illuminated fields. 
2. Apparatus whereby intensity of one or both fields may be varied 
according to known law. 
3. Standard source of light. 
Varying the intensity. 
Distance. 
Effect of area of sources. 
Apparent candle-power. 
Sector disc. Talbot’s Law, Napoli, Brodhun, Hyde. 
Diaphragm. Cornu’s cat’s-eye. Lens. Diffusing plate. 
Polarization. 
Inclined plate. 
Varying source—height of flame, voltage. 
Sight-box. 
Fields must be contiguous or adjacent. 
Equality principle. Contrast principle. 
Lambert or Rumford. 
Bouguer-Foucault. 
Wedge. 
Elster-Joly block. 
Bunsen. © 
Grease-spot. Disappearance. Contrast—Rudorff mirrors. Lee 
son built-up disc. Theory. Construction. Errors in use. 
Limitations, Accuracy. 
Lummer-Brodhun. 
Plain. Contrast. Sensibility. 


412 ILLUMINATING HNGINEERING 


Practical apparatus. 
Precision bar photometer. Scales, equal part, proportional, direct 
reading. 
Industrial: gas, electric. 
Portable and illumination photometers. 
Weber, Martens, Blondel, Sharp-Millar. 
Auxiliary apparatus. 
Lamp rotators. 
Distribution, elevating lamp, three mirror. 
Arc-lamp apparatus. Long arm. 
Integrating and summation apparatus. 
Blondel, Matthews, sphere. 
Heterochrome photometry. 
Equality of contrasts—Leeson disc. 
Visual acuity. ' 
Flicker photometer. 
Rood, Simmance-Abady, Whitman, Schmidt & Haensch. 
Spectro-photometers. 
Vierordt, Lummer-Brodhun, Nichols, Brace. 
Three-color apparatus. Ives colorimeter. 


Lecture [ 
Definition and Scope 

Photometry is broadly defined as the science of the measurement 
of light. Ordinarily the name has been used to refer to the meas- 
urement of the intensity of sources of light, since this has been 
the measurement most commonly made. The measurement of il- 
lumination as distinct from intensity of a source has come into 
much greater prominence in recent years, and the term “ illumi- 
nometry ” has been used for this class of measurements. Hssen- 
tially, there is no difference between illuminometry and photometry, 
all photometric measurements being essentially measurements of 
illumination or brightness; hence, we may say that the term il- 
luminometry includes the term photometry. The term photometry, 
however, is very much preferable, and is properly used to include 
all the branches of the measurement of light and illumination. 


Quantities Measured 


The fundamental quantity with which photometry has to deal is 
luminous flux. The intensity of a source is its flux per unit solid 
angle. The illumination is flux per unit area. These three quan- 
tities, flux, intensity of a source and illumination, are the chief 
ones with which photometry has to do; while specific intensity— 


THE MEASUREMENT OF LIGHT 413 


specific flux, etc.—are also quantities included in the ordinary 
scope of photometry. 


Measurements 


The Eye as a Photometric Instrument. The normal human eye 
being the only instrument which is sensitive to light, in as far as 
ght concerns, the normal human being, it is the eye which must 
constitute the fundamental photometric instrument. The eye by 
itself is incapable of determining with any accuracy the intensity 
of a source of light or the intensity of illumination. Moreover, 
the eye is incapable of forming any correct estimate of how many 
times one light is brighter than another. It is only by the use 
of special methods that the eye is adapted to photometric work. 
These methods depend upon the following properties of the eye: 

First. The eye is capable of determining with a considerable 
degree of nicety the equality of the brightness of two contiguous 
illuminated fields. With special devices the difference in brightness 
which can be detected by the eye is quite small, therefore photo- 
metric measurements may be made by a zero method relative to 
a standard of luminous intensity or of illumination. 

Second. Any given eye under given conditions is capable of de- 
tecting a certain degree of contrast with a certain illumination, or 
of just distinguishing certain objects with a certain illumination ; 
for instance, a certain minimum illumination is required with a 
given eye in a given condition to enable a certain print to be read. 
This point is not very well defined, but is sufficiently well defined 
to enable photometric measurements of a certain class to be made 
in accordance with the principle involved. This is called the 
“ visual-acuity ” method. There is also a zero method dependent 
on the disappearance of flicker. This will be discussed in its proper 
place. | 

By the zero method where the eye is comparing the brightness 
of one field with that of another, and deciding when they are equal, 
difficulty is encountered whenever the illumination of the two fields 
differs in color. Color differences represent differences in quality, 
and, from a theoretical point of view, substances which differ in 
quality cannot directly be compared quantitatively. The practical 
effect of color difference in photometry by the zero method is to 
make the error of measurement considerably greater, and to give 
rise to personal differences between different individuals who ap- 


414 _. JLLUMINATING ENGINEERING 


praise the different colors according to different personal standards. 
Thus color differences constitute one of the greatest inherent diffi- 
culties in ordinary photometry. In using the second or the minal — 
method referred to above, color differences are eliminated, since 
only one color is observed at a time. The illumination observed 
by this method is given a value proportionate to its usefulness in 
enabling objects to be distinguished. This value may differ con- 
siderably from that obtained by the zero method. 
In the flicker method color differences are eliminated. 


Methods—Direct Comparison and Substitution 


There are two general methods employed in the use of photo- 
metric apparatus. In the dvwrect-comparison method the appa- 
ratus is set up in such a way that the source of light to be measured 
is compared directly with the standard source, one being placed on 
one side of the photometric apparatus and the other on the other, 
and the balance secured. In making measurements after this 
method, many precautions are required to eliminate the errors. due 
to one-sidedness of the apparatus, or to a tendency of the observer 
to favor one side rather than the other. In working by the sub- 
stitution method, the comparison between the source of light to be 
measured and the standard is indirect. The procedure is, first, to 
set up the standard source of light and compare with it a constant 
source of light of convenient intensity. Then the standard source 
of light is removed and the unknown source is substituted for it. 
The unknown source is then compared with the constant inter- 
mediate source of light, and its value in terms of the standard is 
computed from the two sets of measurements.. This method of 
procedure has the advantage over the direct-comparison method 
that all errors due to lack of symmetry in apparatus, etc., are 
eliminated. ‘The substitution method is to be preferred to the 
direct-comparison method in the great majority of all cases arising 
in photometry. 


Apparatus for Zero Method 


Any apparatus for making photometric measurements according 
to the zero method, that is, by balancing the brightness of two 
adjacent or contiguous fields, consists essentially of the following 
elements: First, an arrangement by which the two adjacent or 
adjoining fields are obtained, one of the fields being illuminated 
by the standard light and the other by the light to be measured. 


THE MEASUREMENT OF LIGHT 415 


Second, in an arrangement by which the intensity of the illumina- 
tion of one or both the fields may be changed according to some 
known law until equality is secured. Third, a standard source of 
light. 

The apparatus by which the contiguous fields are obtained is 
called the sight-box or photometer head, or, for short, the photome- 
ter. Properly speaking, the photometer includes the whole appa- 
_ ratus, but the-distinction here noted is in many cases a convenient 
one to make, and no confusion should arise because of the use of a 
term proper to the whole apparatus for a part of the same. 

The question of a standard source of light is a separate one which 
has been treated by another lecturer. 

Apparatus for Varying the Illumination 

Variable Distance. The simplest and most common way to vary 

the illumination on the photometer disc in a known manner is to 
vary the distance between the photometer disc and the source to 
be measured. For point sources the illumination produced is in- 
versely proportional to the square of the distance between the 
source and the illuminated surface, the illuminated surface being 
placed at right angles to the rays. Hence, by varying the distance 
of either of the sources of light, or by moving the photometer into 
some position along the straight line adjoining the sources, the 
desired equality of illumination may be obtained. The mathe- 
matical relations are as follows: 
If E is illumination on the two fields of the photometer, I is 
the candle-power of the unknown source, I’ the candle-power of 
the comparison lamp, r the distance between unknown lamp and 
the photometer disc, and r’ the corresponding distance for the com- 
parison lamp, 


or 


The most common arrangement is to set the lamps to be measured 
at the extremities of a straight horizontal track or bar. On this 
bar is a carriage to which the photometer head is attached. The 
carriage is moved along between the lights until the desired equality 
is obtained. The results are computed according to the formula 


2 
aes i Pi 
I=] r” 





416 ILLUMINATING ENGINEERING 


in which I and I’ are the intensities of the two sources of light, and 
r and r’ the distances between the respective sources and the pho- 
tometer disc. 

It is not infrequently desirable to alter the distance between only 
one lamp and the photometer disc. For instance, the photometer 
may be stationary, and the distance of the comparison lamp may 
be adjusted to give equal illuminations. In this case the product 
I’r? does not change, and the formula for the photometer is 

To cone 
Or the comparison lamp and the photometer carriage may be fas- 
tened rigidly to each other so that the distance r’ is constant and 
the distance r varied. In this case the illumination on the pho- 
tometer disc is constant at all times, a feature which has some 
advantages. The formula in this case becomes 
I=constant .<r?. 

A further modification of the variable distance method is an 
arrangement wherein the length of the path of light is varied by 
moving a mirror or pair of mirrors set at right angles to each other. 
The photometric law remains the same. | 

Limitations of Variable-Distance Method. In employing the in- 
verse square law it is necessary to remember that it applies in all 
strictness only to point sources of light, and that for sources of 
linear dimensions large in comparison with the distance at which 
they are measured, the law does not apply. This is due to the 
fact that an element of the luminous body which is not situated in 
the line normal to the photometer disc sends rays to the dise which 
impinge upon it at an angle other than 90°, and consequently 
produce a smaller illumination than if they fell normally. More- 
over, the angle of emission of these rays from the luminous surface, 
supposing the luminous surface to be parallel with the photometer 
disc, is not 90°. ‘These two effects, according with Lambert’s 
cosine law, produce a diminution in the illumination. It is there- 
fore necessary that the angle at the photometer disc subtended by 
the source of light should be below a certain limit. The rule has 
been given that the linear dimensions of the source of ight should 
not be over five times the distance between the source of light and 
the photometer disc. It is safe, and usually entirely convenient, 
to keep far within the limitations of this rule. 


THE MEASUREMENT OF LIGHT AA? 


It is necessary also to see that the angle of incidence of the light 
upon the two fields of the photometer is the same. If the angle is 
different on one side from what it is on the other, an error will be 
introduced according to Lambert’s cosine law. Any such error as 
this, however, may be eliminated in the substitution method. 

Apparent Candle-Power. It is frequently convenient to express 
the photometric properties of a combination, such as a lamp with 
a reflector, or a very extended source of light, in terms of the 
candle-power of a point source which would produce at a given 
distance the same illumination as the arrangement to be measured 
produces. For example, the law of inverse squares cannot be as- 
sumed to hold for a lamp with a concentrating reflector within rela- 
tively short distances from the lamp. However, for purposes of 
illumination computation, it is important to know what the equiva- 
lent candle-power of the combination is at some practical distance. 
To this quantity the term “ apparent candle-power ” is applied, the 
distance at which this apparent candle-power is measured being 
also specified. In reflector measurements the apparent candle- 
power at a distance of 10 feet is commonly given. This means 
merely that when the lamp and reflector are measured with the 
photometer 10 feet away the illumination which is produced is 
equivalent to that of a lamp alone having the candle-power given. 

Rotating Sector Disc. If an opaque disc from which equally 
spaced sectors of definite angular dimensions are cut is placed in 
the path of a beam of light and rotated rapidly, the amount of radi- 
ation passing through the open sectors bears to the total radiation 
the same ratio that the angular aperture of the open sectors does to 
360°; that is, if the open sectors aggregate 36° in aperture, 10 
per cent of the radiation will pass through. If the light so dimin- 
ished falls upon a screen, and the rotation is sufficiently rapid, the 
eye will observe the screen uniformly illuminated, and the impres- 
sion made upon the eye will be, in accordance with Talbot’s law, 
the same as if the same flux of light fell upon the screen in a- 
steady stream as actually falls on the screen in the intermittent 
_ stream transmitted by the disc. Therefore, physiologically, as 
well as physically, the beam transmitted by the rotating disc varies 
as the ratio of the angular aperture of the open sectors to the 
total periphery of the disc. 


418 ILLUMINATING ENGINEERING 


Verification of the Law of the Disc.* By a series of careful ex- 
periments, Lummer and Kurlbaum have shown that for lights of 
the same color Talbot’s law held for the dise within the errors of 
observation. . 

As a result of experiments by Ferry ¢ doubt had been cast on 
the validity of Talbot’s law when lights of different color are com- 
pared by means of the sector disc. This question has been investi- 





Fig. 1—Sector Disc with Fixed Apertures. 


gated by Hyde,t whose careful experiments have shown that Tal- 


bot’s law applies to the rotating-dise method within the errors of 
observation, both when lights of the same and different colors are 
compared with all apertures of the disc from 15° to 240°. 


* Zeitschrift fur Instrumentenkunde. Elektrotechnische Zeitschrift, 
Aug., 1896. . 

7 Phys. Rev., Vol. 1. 

t Bull. Bureau of Standards, Voi. II, p. 1: 


THE MEASUREMENT OF LIGHT 419 


Practical Forms of Sector Disc. The-sector disc, which is a most 
important adjunct in photometric work, can be made either with 
fixed openings or with: variable openings. With fixed openings, 
it is convenient as a means for-reducing the intensity of a beam 
of light in a known ratio, an operation which ‘is often desired in 
order to bring a given measurement within the range of a given 
photometer bar. A fixed disc of this sort, as used by the Bureau 
of Standards, is illustrated’ by Fig. 1. Evidently one motor may 






<ez 





Fig. 2.—Sector Disc. | 


be supplied with a series of discs, so that a variety of ratios are 
obtainable, but in any case the fine’ variations of photometric set- 
tings must be made by some other means. A dise may be con- 
structed to produce any required diminution from 50 per cent 
downward by taking two equal metal discs, out of which equally. 
spaced sectors are cut, of such dimensions that the open sectors 
occupy one-half of the disc. These are mounted face to face on a 
shaft, and are provided with a clamp to hold them together in any 
position. By sliding the discs over each other, the amounts of 
the open sectors of the combined disc may be varied at will, and 
the ratio may be read from a graduated scale on one of the discs. 
17 


420 ILLUMINATING ENGINEERING 


In the construction shown in Fig. 2, the area of the open ‘sectors 
may be varied while the discs are in full rotation, thereby consti- 
tuting a device by which complete photometric settings can be 
made. The sector disc D is mounted on the axis A, while a sim- 
ilar disc D’ is fastened to the hollow sleeve A’, fitting over the axis 
of the first disc and rotating with it. A’ is pierced with the spiral 
slot S, while A has a longitudinal groove of the same width. A 
hollow sleeve V fits over the sleeve A’ and carries a pin which 
passes through the spiral slot and terminates in the longitudinal 
groove. A longitudinal movement of V, which can be effected by 
means of a lever or a micrometer screw when the discs are rotating, 
displaces the one disc with respect to the other, and varies the 





Fig. 3.—Brodhun’s Sector. 


effective aperture of the combination. The lever or micrometer 
screw is calibrated to show the ratio of the disc and can be read 
- without stopping the disc. : 
Brodhun’s Variable Sector. Brodhun has not only constructed 
a variable rotating-sector disc, in which by special optical arrange- 
ment the actual angle between the sectors can be read from the 
disc while rotating, but he has also produced another and much 
simpler apparatus for changing the intensity according to Talbot’s 
law. In the latter apparatus the variable sector remains fixed, 
while the beam of light is caused to rotate about it. The arrange- 
ment is shown in Fig. 8. The beam of light striking the Fresnel 
prism P is twice reflected to the Fresnel prism P’ on the opposite 
side of the sector D, by which it is returned to its original axial 
direction. The prisms are rotated rapidly, and the photometric 
setting made by the aid of the adjustable sector dise D, the position 
of which, since it is stationary, can be read at once from an affixed 
scale. In the form in which it is constructed by Schmidt & 


THe MEASUREMENT OF JAGHT 421 


Haensch, this apparatus is adapted to the measurement of light 
in rather small beams. ‘There is no reason, however, why the 
principle should not be applied to a larger apparatus made with 
mirrors instead of prisms. 

Hyde’s Variable-Sector Disc. For the special purpose of spectro- 
photometry, in which the beam to be photometered enters the nar- 
row slit of a collimator, Hyde has produced a very simple form of 





Fic. 4.—Hyde’s Sector. 


variable-sector disc. In this form (Fig. 4) the apertures of the 
disc are not straight and radial, but are curved in such a way that 
near the center of the disc the apertures are nearly 100 per cent, 
and the aperture varies from that to zero at a point near the cireum- 
ference of the disc. It is evident that if a slit which is to re- 
ceive light is placed so that it hes at right angles to the axis of 
the disc, the amount of light which it will receive will vary with the 
position of the disc with respect to it. That is, when the beam 
of lhght reaching the slit passes through the openings near the 
Lit 


422 ILLUMINATING ENGINEERING 


center, the diminution introduced by the rotating dise will be small. 
This diminution can be increased steadily by a lateral motion of the 
disc. If the relation of the disc carrier, with respect to the slit, is 
fixed in the apparatus, the position of the disc, as indicated on a 
scale with vernier, will give, by a previous calibration, the per- 
centage of light which the disc is transmitting. 

Use of Diaphragms. A diaphragm may be used in several ways 
as a means for diminishing the intensity of a beam of light. If, 
for example, the source of light is a uniformly illuminated dif- 
fusing surface, the amount of light which it emits varies directly 
with its area, so that if a diaphragm is placed before it the light 
emitted will vary directly as the area of the opening of the dia- 
phragm; or, if a converging lens is so placed that an image of 
the bright surface which is the source of light is thrown by it on 
to the photometer screen, the flux of the beam may be diminished 
by stopping down the lens, and the intensity will vary very nearly 
proportionally to the aperture of the diaphragm. The greater 
thickness of the lens toward the center, as compared with the 
sides and the possible aberration of the lens, will cause this law 
to be not quite rigorous, and any such arrangement as this needs 
to be calibrated by experimentation. With either of these arrange- 
ments the diaphragm may be one which can be adjusted contin- 
uously, whereby a convenient and effective device is constituted. 
Ordinarily, the bright surface which constitutes the source of light 
will be a piece of translucent glass. Ground glass should not be 
used for this purpose, since it is a very poor diffuser. Some of the 
other forms of glass, such as alabaster glass, etc., should be used, 
and it is preferable that the surface of such glass shall be ground 
as an additional precaution. 

The diaphragm principle may also be used in connection with 
straight-filament incandescent lamps. If the image of the filament 
is thrown by means of a lens on to an adjustable slit, with the 
image crossing the jaws of the slit at right angles, the light trans- 
mitted by the arrangement will vary directly as the width of the slit. 

Any good form of adjustable diaphragm can be used for photo- 
metric work. The one most commonly employed is Cornu’s “ cat’s 
eye,” which consists of two metal strips pierced with rectangular 
openings, and arranged to slide one upon the other in the direction 
of the diagonal line of the openings. The movement may be pro- 


Tur MEASUREMENT OF LIGHT 423 


duced by a rack and pinion, or by a micrometer screw, and the 
position may be read from an attached vernier and scale. The 
metal strips are illustrated in Fig. 5. 

Another available form of diaphragm is the iris diaphragm, which 
is very commonly used with photographic lenses. The calibration 
of such a diaphragm is made empirically. 

Polarization.* If the light from one source is polarized by pass- 
ing through a Nicol or other polarizing prism, or by reflection from 
a pile of glass plates at the angle of polarization (about 56° 20’ 
for light crown glass), it loses more than one-half of its intensity 





Fic. 5.—Cornu’s Cat’s Eye. 


in the process, and the intensity of the polarized beam may be still 
further cut down to any extent by means of an analyzer. This 
analyzer may be a duplicate of the polarizer, or it may be any form 
of totally polarizing device. When the polarizer and the analyzer 
are “parallel,” the polarized light emerges from the analyzer but 
little decreased in intensity. When they are “crossed” the beam 
is entirely extinguished. The intensity of the beam varies as the 
square of the cosine of the angle of rotation of the analyzer with 


* The reader is referred to the subject of polarization in any good 
text-book of physics. 


4.24 ILLUMINATING ENGINEERING 


respect to the polarizer. The method must be used with precau- 
tion where there is any possibility that the light which is to be 
measured is already polarized partially, as for example, light from 
the sky. The Nicol prism suffers from a further disadvantage ot 
being very expensive in large sizes and absorbing a very consid- 
erable percentage of the incident light, thereby producing a dark 
field. Moreover, its absorption is selective, being very great in the 
blue and violet end of the spectrum. On account of these dis- 
advantages, and since the required end can usually be attained by 
simpler means, the polarization method is not very extensively used 
in photometry. 

Absorbing Media. ‘The intensity of a beam -of-light may be cut 
down in known ratio by passing it through an absorbing medium. 
A prime necessity in the case of such media is that they shall be 
uncolored; that is, that they shall transmit all colors of light 
equally. This is a condition which is scarcely ‘fulfilled to an exact 
degree by any medium, but various media are available which are 
sufficiently colorless for practical purposes. For diminishing the 
light to a slight degree, a plate of clear glass may be used, or 
several plates may be piled one on the other. By inclining these 
plates to the axis of the beam, the amount of diminution may be 
changed. The diminution in this case is produced chiefly by re- 
flection from the surfaces of the plate. For a glass of known 
index of refraction, the light reflected on one surface, the incidence 
being normal, the coefficient of reflection may be computed from 
Fresnel’s equation, 

nosy 
atest, 
where n is the index of refraction. For example, with light crown 
glass having an index of refraction of 1.5, the value of the beam 
transmitted from the air into the glass normally is 96 per cent of 
the incident beam. A further reflection of the same percentage 
of the beam which remains takes place on emerging from the glass 
into the air, so that the total light transmitted is 96 per cent by 
96 per cent, or 92.2 per cent plus such light as is regained by 
secondary reflection. 

Absorbing media may be divided into two important classes. 
First, those media which permit the beam to pass unaltered, except 
in intensity; second, those which diffuse the light as well as ab- 
sorbing it. An example of the first class of absorbing media is 


THE MEASUREMENT OF LIGHT 425 


ordinary smoked glass. An object can be seen through a piece 
of smoked glass without any distortion, only with a diminution of 
the brightness. An example of the second is a piece of alabaster 
glass or of thin paper, that is, media which diffuse as well as absorb 
the light, and which are commonly called translucent. The action 
of the media of the two classes in photometric apparatus is quite 
different. A piece of smoked glass can be interposed between a 
lamp and a photometer at any point in the beam, and will cut 
down the light incident upon the photometer by a definite amount. 
_ If a diffusing glass is used for this purpose, it becomes a secondary 





Fic. 6.—Photometric Wedge. 


source of light, and the amount of diminution found on the pho- 
tometer disc differs greatly with the position of the diffusing glass 
with respect to the disc, and if the glass is stationary on the pho- 
tometer bar while the photometer is moved, photometric distances 
must be measured from it rather than from the lamp which is the 
actual source of light. 

The Photometric Wedge. By _ a wedge-shaped piece of 
smoked glass, the intensity of a beam of light can be diminished 
continuously. This was done formerly by Pickering. An im- 
provement in the photometric wedge was introduced by Spitta,* 
who used two wedges to slide over each other instead of a single 
wedge, as illustrated in Fig. 6. With this arrangement, the thick- 
ness of the absorbing medium through which the light must pass 


* Proceedings of the Royal Society of London, Vol. 47, p. 15, 1889. 


426 ILLUMINATING ENGINEERING 


can be varied from a lower limit, which depends upon the acuteness 
of the angle of the wedge and upon the width of the opening in 
the diaphragm through which the light is allowed to pass, to an 
upper limit, which is nearly twice the thickness of one wedge. The 
loss of light in passing through an arrangement of this sort is due 
to two effects: first, absorption in the wedges; second, reflection 
from the surfaces of the wedges. The second loss enters in as a 
constant quantity, superimposed upon the absorption loss, which is 
proportional to the thickness of the wedge. On this account, and 
because of inequalities in the glass, it is necessary that wedges 
should be calibrated throughout their entire range before being 
used in photometric measurements. The relative position of the 
wedges may be read from a vernier and scale attached to them. 

Another form of graduated absorbing medium has been employed 
in a portable photometer by Dr. Williams,* who used a photographic 
film which had been exposed and developed so that it showed a 
gradually increasing density. Evidently this plan is capable of 
considerable development. An arrangement of this kind must also 
be calibrated empirically throughout its length. 

Inclined Plate. If the source of light is a diffusely reflecting 
or transmitting surface, as, for instance, an illuminated surface of 
plaster of Paris, or a window of diffusing glass, the light which 
it sends in a given direction may be altered by changing the angle 
between the normal to the plate and the direction in question. If 
the diffusing surface is a good one, the diminution of light as 
the plate is turned from the normal will vary proportionately to 
the cosine of the angle for considerable angles from the normal. 
In any case, it is advisable that a plate used in this way should be 
calibrated empirically. 

Varying the Source of Light. In certain apparatus the photo- 
metric setting has been made by varying the total amount of light 
given by the comparison source. With a flame source this may 
be done by raising and lowering the flame. Then a measurement 
of the flame height indicates the photometric setting. When an 
incandescent lamp is used as the comparison source, its intensity 
may be varied over considerable limits by varying the impressed 
voltage. This has the disadvantage that the color of the light varies 
at the same time. The first of the above methods is adapted to 
only the roughest kind of work. 


* Transactions Illuminating Engineering Society, p. 540, 1907. 


THE MEASUREMENT or LIGHT 427 


PHOTOMETERS 
Principles 

In the fields of photometric sight boxes two principles are made 
use of. First, is the equality principle. In photometers intended 
to employ this principle the fields are so constructed that the eye 
compares their brightness directly, and endeavors to tell when they 
are equally bright. It is very difficult to do this with any degree 
of accuracy unless the fields are so arranged that the dividing line 
between them disappears when the equality point has been reached. 
Hence, such photometers are called also “ disappearance ” photome- 
ters. ‘Then the eye, by observing the merging of one field into 
another, can determine the point of equality with considerable 
accuracy. Second, is the contrast principle. In accordance with 
this principle, each field consists of two parts: first, a part il- 
luminated by its own proper source; second, a part illuminated 
by the other source, but to a different degree. With this arrange- 
ment equality exists when the contrast in the right-hand field be- 
tween the portions of the field illuminated by source A and source 
B is the same as the contrast in the left-hand field between the 
portions illuminated by source B and source A. If the degree of 
contrast is the correct one, the eye is able to determine this equality 
of contrasts with great precision. 

In photometers using the contrast principle, the equality prin- 
ciple may also be employed, since when the equality of contrast 
is established equality of illumination is also observed. If, when 
equality of illumination is observed, exact equality of contrast 
is not observed the construction of the apparatus is faulty. The 
degree of contrast which gives the most sensitive arrangement de- 
pends to some extent upon the illumination of the fields. The © 
contrast must appear in all cases quite small. Since the ability 
of the eye to distinguish a contrast varies with the brightness of 
the field, a smaller contrast should be employed with a photome- 
ter used with very bright fields than with one where the fields are 
only faintly illuminated. As a practical matter, a contrast of 
about 8 per cent between the two portions of the field seems to 
give in a general case the most satisfactory results. 

A great variety of photometers have been constructed, embody- 
ing the above principles, and other variations are possible. At 
the present time only a few of the varieties are of much practical 


428 ILLUMINATING ENGINEERING 


importance, so that while some of the older kinds should be men- 
tioned as being of interest and of use in certain special cases, par- 
ticular attention will be given to the description of those forms 
which have proved themselves to be of the greatest practical im- 
portance. | 

Lambert’s Photometer. This photometer, of which Rumford’s 
photometer is a modification, dates from the middle of the 18th 
century. The opaque screen HI (Fig. 7) is so arranged before the 
white screen BCEG that a certain region to the right of the line 
DF is illuminated only by the source L; while a region to the left 
of the line is illuminated only by L’. When the two fields are 





Fig. 7.—Lambert’s Photometer. 


made equal by ati the distances of the lights, the line of 
separation along DF disappears, and the two fields merge intd one. 
In order not to introduce any error due to difference in obliquity 
of incidence, the angles of incidence must be equal. 

Bouguer’s Photometer differs from Lambert’s in that the opaque 
screen dividing the fields is placed perpendicular to the illumi- 
nated screen, as is shown in Fig. 8. 

Foucault’s Photometer differs from that of Bouguer in ee the 
screen SS’ is of translucent diffusing material, such as-a layer of 
fine-grained precipitated starch. between plates of glass, and the 
fields are viewed from the far side. ‘The screen HI is made mov- 
able lengthwise so that the edges of the fields can be made exactly 
to touch each other. When the fields are equal, the line of separa- 
tion along I vanishes. The screen is viewed either through a 


THE MEASUREMENT OF LIGHT 429 


simple tube or through a telescope. The sight box is placed at the 
junction of two tracks meeting at an acute angle, along which the 
lights can be moved. 

Harcourt’s Table Photometer, as adopted by the London Gas Ref- 
erees, is a modified form of Foucault photometer. The separating 
screen at right angles to the diffusing screen is, however, replaced by 


S 





Fie. 8.—Bouguer’s Photometer. 


an opaque screen set at a little distance from the diffusing screen and 
parallel with it. In the opaque screen is a square hole. The two 
sources of light shining through this opening throw bright spots 
on the diffusing screen. The screen is so adjusted that these spots 
are made to touch each other. The equality of brightness of these 
two spots is observed looking from the rear side of the diffusing 
screen. 


L 





Fig. 9.—Ritchie’s Wedge Mirrors. 


Ritchie’s Photometer has two mirrors M and M’ (Fig. 9) which 
throw the light from the two sources upon the translucent screen 
SS’. This sight box is mounted on a track between the lights, and 
is moved along until equality of illumination is secured. 

Nichols has modified this photometer by arranging the mirrors 
to cross at their mid-points (Fig. 10), one above the other. The 
illuminated fields are thus seen one above the other, instead of side 
by side as in in the original form. 


430 ILLUMINATING ENGINEERING 


Ritchie’s Wedge Photometer. This differs from the form de- 
scribed above in that the screen SS’ is removed, and for the mir- 
rors are substituted pieces of white cardboard with unglazed sur- 
faces. When a setting is secured with lights of the same tints the 
line of separation between the cardboards disappears, provided 
that it is a sharp one. Various modifications of this photometer 
have been described. If the wedge is made of plaster of Paris, and 
the edge of it is made very sharp, the photometer is said to give 
very good results. Unless the wedge is placed directly perpen- 
dicular to the track, the wedge photometer is subject to an error 
due to the unequal obliquity of the surfaces with respect to the 





Fig. 10.—Nichols’ Crossed Mirrors. 


axis of the photometer bar. A small variation from the proper 
position of the wedge may produce an error of noticeable dimen- 
sions because the effect of a variation is to increase the obliquity 
of the light on one side of the wedge and to decrease it on the 
other, thereby producing a double effect. Moreover, the angle of 
the wedge is of such a value that the cosine is varying rapidly. 
Since the illumination of the plate also varies with the cosine of 
the angle according to: Lambert’s law, this condition is such that 
a small variation in the position of the wedge will produce a rela- 
tively large error in the result. 

Elster-Joly Block. ‘This simple photometer has been consetoree 
in a variety of ways, all in accordance with the same principle. 
Two blocks of translucent substance, such as paraffine or milk- 


THE MEASUREMENT oF LIGHT 431 


glass, are placed either side by side or one over the other in such 
a way that each receives the light from one of the lamps. For 
instance, if they are placed side by side, a very thin metallic dia- 
phragm may be placed between them. When viewed from the 
front each block is seen illuminated by the internally diffused 
light from its respective lamp, and the equality of brightness is 
readily observed. 

Bunsen Photometer. This is one of the most common and the 
most practical form of photometer in use at the present day. The 
photometer is capable of considerable variations in the manner 
of its construction and use, the principle involved being always the 
same. The theory of it may be outlined as follows: Suppose a piece 
of white paper, a portion of which has been rendered translucent 
by a drop of grease or of wax, is held between two sources of light 
which are to be compared. 

If an observer looks at the right side of the paper while holding 
it in such a position that the illumination of the paper due to the 
source at the right is considerably greater than that due to the 
source at the left, he will see the grease spot as a dark spot on a 
light background. If the paper be held so that the illumination 
due to the source at the left is enough greater, the grease spot 
will appear bright on a darker background. At some intermediate 
point, consequently, the spot, being neither brighter nor darker 
than its background, will disappear altogether, unless the quality 
or color of the light coming from the greased portion is different 
from that coming from the ungreased portion. If this difference 
in quality is but small, the point of equal illumination of the two 
parts of the screen can be determined with considerable nicety. 

This point does not in general represent, however, the position 
of equality of illumination on opposite sides of the disc, for if the 
observer makes a similar setting while looking at the left-hand 
side of the disc, a different point of disappearance of the spot will 
be found. This is due to the fact that the amount of light lost by 
absorption in the greased portion is different from that in the 
ungreased portion, and to the different diffusing powers of the 
- greased and the ungreased portions. The true photometric set- 
ting can, however, be deduced from a pair of observations such as 
the above. If we designate by r, and r, the distances to the re- 
spective sources when the setting has been made, while observing 
the side of the dise which is turned toward the source of intensity 


4.32 ILLUMINATING ENGINEERING 


I,, and by r,’ and r,’ similar quantities when the disc has been 
reversed and the same side of it is observed turned toward the 
source of intensity I,, then, 

ape eae! I. 

1 Dats 2 

For, designating by p, and t, the coefficients of reflection and 

transmission of the ungreased portion of the disc, and by p, and t, 
the same for the greased portion, the condition that. the brightness 
of both parts shall be equal when viewed from the side turned 
toward I,, or that the line of demarkation shall disappear, is ex- 
pressed by the equation 


ee ik i i 
niet Tr? = Po my ca ght ee i 





Similarly, when S cise is le tity ean the same side is again 
observed, 





pte +t, is = po ae +t, a 
These equations yield the following values of I: 
[ 2a: t.—t, Ti if 
: Piz Pawsley GEM 





12 
IL= ate in I,, | 
Multiplying these equations together and extracting the square- 
root of the product, we have, as above, 
3 Tare 
1 Pa aD 
The equations become more complicated, in case the dices is not 
reversed for the setting to right and left, since the coefficients 
p and t are usually different on opposite sides of the same disc. 
Evidently the Bunsen photometer can be used without reversal 
if the substitution method be followed. In this case the ratio of 
the intensities of the lights is given directly as the squares of the 
distances. 





The Bunsen as a Contrast Photometer _ 


It is far more common, as well as far better arernee in the use 
of the Bunsen photometer to observe both sides of the disc simul- 
taneously by the aid of a proper arrangement for the purpose. 
When this is done, the observer sees a contrast between the greased 
and ungreased portion of the disc; that is to say, with a given 


THE MEASUREMENT OF LIGHT 433 


illumination, the brightness of the greased and ungreased portions 
will not be the same. The photometer is adjusted on its bar until 
the contrast between the greased and ungreased portions is the 
same on one side as on the other. So used, the Bunsen photome- 
ter becomes a contrast photometer, and is much more sensitive 
than when used as a disappearance instrument. 

The opposite sides of the disc will usually have slightly different 
optical properties, so that it is necessary, in order to get a true 
setting by the direct-comparison method of photometry to take 
readings with the disc direct and reversed. If the dise is a good 
one, the readings in the two positions will lie very close to each 
other, and the mean position, as obtained by averaging, may be 
taken as the correct setting of the photometer. If the positions 
do not lie close together, the equation for computing the candle- 
power of the unknown source of light is the same as that given 
above for the Bunsen disc used as a disappearance photometer, 
namely, 

ce rr,’ a 

The derivation of this stritdtion for the contest method is analo- 
gous to its derivation as given above for the disappearance method. 
The complication of reversal tee, be avoided by using the substi- 
tution method. 

‘Construction of Dises. The method employed in constructing 
- the disc has a great influence not only on its sensitiveness, but 
also onthe accuracy of the results obtained by its use. Three 
conditions must be fulfilled by a good disc; 1, the two sides must 
be alike; 2, the contrast between the greased and ungreased por- 
tions must be such as to give the proper degree of sensitiveness for 
the work in hand; 3, the paper and the grease must exercise no 
selective reflection or absorption on the light. It is a matter of 
great difficulty to secure uniform discs. If the paraffine or stearine, 
which are the materials commonly employed in making the spot, 
are in excess on the surface the properties of the disc may be 
materially changed by changing the degree of polish on the sur- 
face of the wax. The degree of contrast obtained varies greatly 
with the amount of wax which the paper has absorbed. 

Various procedures have been recommended for constructing 
discs. A piece of copper, round- or star-shaped, as it is desired 
that the dise shall be, may be plunged into a bath of the molten 





434 ILLUMINATING ENGINEERING 


wax, held there until it is thoroughly warmed, and then pressed 
on the paper until the wax has completely penetrated it. The ex- 
cess of wax should be removed by scraping or by laying on a piece 
of blotting paper and pressing with a hot iron. Another way is to 
clamp the paper between pieces of metal having apertures of the 
shape that the spot is to assume, and to dip the whole into the 
molten wax. ‘Two equal cylindrical pieces of metal may be em- 
ployed, in which case the disc has an opaque center and a translu- 
cent margin. After scraping off the excess of wax, the disc so 
prepared may be placed in a steam or a warm-air bath until the 
distribution of the wax has become uniform; very good discs can 
be made in this way. 

Heavy, unsized white paper should be used. White drawing 
paper makes good discs. 

The Leeson Disc. ‘This is a built-up disc, made by pasting sheets 
of thin, translucent paper to each side of a piece of heavy paper 
in which an aperture, in the form usually of a slender-pointed 
star, has been cut. ‘The inner paper may be the same as is used 
in the construction of grease-spot discs. 'The outer paper is se- 
lected to give the right degree of contrast, and should have an un- 
glazed surface. Thin starch paste may be used, care being taken 
that the outer sheets adhere to each other over the entire portion 
where the middle paper is cut away, particularly in the points of 
the star. The outside and the middle papers should be wetted 
before the paste is applied, and the excess of water removed by 
pressing between cloths. After pasting, the dise should be clamped 
tightly between blotting paper in a letter press, where it is allowed 
to remain until it is thoroughly dry. Such. a dise is sometimes 
mounted between sheets of clear, white glass, which protect it from 
dust and counteract any tendency to buckle out of shape. This 
disc possesses the advantage that its surface is everywhere of the 
same texture, and that the desired degree of contrast can be ob- — 
tained by properly selecting the materials. It is also much easier 
to secure a good degree of uniformity in making these discs than 
in the case of the grease-spot disc. 

Selection of Discs. Bunsen photometer discs differ in sensi- 
bility far more than is commonly recognized, hence it is important 
to make a careful selection of the dise which is to be used. A sensi- 
tive disc is one which will show a marked change in appearance 
for a small change in the illumination on either side. If the pho- 


THE MEASUREMENT OF Licut A35 


tometer is set at a balance, a small motion to one side will cause 
one of the spots to disappear, and a small motion to the other 
side will cause the other spot to disappear. The distance between 
these two disappearance points may be taken as characterizing the 
disc. In a sensitive disc this distance will be small. In an in- 
sensitive disc it may be very considerable indeed. 

Bunsen discs are divided into two classes; that is, positive discs 
and negative discs. In the positive disc, at the position of balance, 
the more translucent portion of the disc appears bright upon a 
darker background. In a negative disc the reverse is true. With 
a positive disc, disappearance of the spot on the right side of the 
dise is obtained by moving the photometer toward the right. With 





Fie. 11.—Bunsen Photometer with Rudorff Mirrors. 


a negative disc the opposite condition obtains. By proper selection 
of papers, built-up discs can readily be made of either the positive 
or negative variety. The positive variety results when the diffusing 
power of the thin paper is large as compared with its absorption. 
The negative variety results when the diffusing power of the thin 
paper is less marked, or when the reflecting power of the thick 
paper used is high as compared with that of the thin paper. The 
proper selection of materials for built-up discs is a matter of 
great importance in determining the sensitiveness of the discs 
produced. 

Riidorff Mirrors. These mirrors constitute a simple device, 
which is in almost universal use, for enabling the two sides of the 
Bunsen disc to be seen simultaneously. It consists in a pair of 
mirrors placed vertically behind the disc and inclined at an angle 
of about 140° to each other. The intersection of the mirrors lies 


436 : ILLUMINATING ENGINEERING 


in the plane of the disc. By looking into the mirrors, two images 
of the disc are seen rather close together, and the eye, by glancing 
quickly from one to the other, can compare them very readily. 
The mirrors themselves should be cut from the same piece of glass. 
They, together with the disc, are mounted in a small box which 
is cut away at the sides to admit the light, and in front to permit 
the screen to be seen, as shown in Fig. 11. ‘The box should be 
painted dead black within and without, and is fixed « on a carriage 
in such a way that it can readily be reversed. 


a oe 


2 





YING 
Yer. 
Fic. 12.—Kriiss Prisms. 


Kriiss Prisms. The disc p (Fig. 12) is greased along a vertical 
line of width ac. ‘The light from the disc, after total reflection 
inside the prisms ABCD, emerges in a direction at right angles 
to the disc, and the latter is seen as shown at the bottom of the 
figure, the two images of the greased portion meeting each other 
sharply in a line. 

Sources of Error. It rarely happens that a dise has exactly the 
same optical properties on both sides. Even if a dise is perfect 
in this respect when first made, the adhesion of dust and slow 
chemical changes are likely to produce differences in the course 
of time. As a consequence, it is necessary when comparing lights 
by the direct rather than the substitution method to reverse the 
sight box, making an equal number of settings in either position. 


THE MEASUREMENT OF LIGHT 437% 


The differences between the sides of a disc may be very large, and 
the neglect of this precaution may lead to serious errors. 

Reference has been made above to the discrepancies in the indi- 
cations of discs due to differences in the thickness of the wax in 
the spot and to the nature of its surface. 

Nichols * has studied another source of error in the use of the 
Bunsen photometer with Riidorff mirrors. He found that different 
observers would set the photometer persistently to the right or to 
the left of its true position, and that the sign of this deviation 
changed when observations were made by the aid of a mirror with 
the back of the observer turned towards the photometer, or when 
the observer worked from the other side of the track. The devia- 
tions became zero when the observers worked with one eye covered. 
The magnitude of this error was in one case 8.7 per cent. The 
effect is to be ascribed to differences in the eyes of the observer, 
and it may be largely avoided by holding the head at a considerable 
distance from the disc, so that both eyes act as one, or by the use 
of Kriiss + prisms which are illustrated in Fig. 25. 

This error is only an exaggerated form of the one-sidedness error 
which is noticed in the case of practically all observers on any 
kind of photometer. The use of the substitution method eliminates 
the above errors in the case of practiced observers who have acquired 
a fixed habit of observation. 

Lummer-Brodhun + Photometer. This is one of the most sensi- 
tive and practical photometers known, and is very extensively 
used, both for precision and for technical photometric work. It 
is made both in the form of an equality photometer and a contrast 
photometer, the latter being the more sensitive device. A scheme - 
of the photometer is shown in Fig. 13. In this figure § is an 
opaque diffusing screen, for example, of plaster of Paris, the oppo- 


* Transactions American Institute of Electrical Engineers, 1889. 

+ Journal fiir Gasbeleuchtung und Wasserversorgung. Vol. 27, p. 587, 
1884. Lente . 
+ It has been pointed out by Prof. Knott in the Philosophical Maga- 
zine, Vol. 49, p. 118, 1900, that a prism combination practically identical 
with the Lummer-Brodhun prism was constructed by Prof. Swan of the 
University of Edinburgh and described by him in the Transactions of 
the Royal Society of Edinburgh, Vol. 22, 1859. Prof. Swan used his 
prism pair in photometric work for many years. In the application of 
the contrast principle to such a prism combination, Lummer and Brod- 

hun may undoubtedly claim priority. 


438 ILLUMINATING ENGINEERING 


site sides of which are illuminated by the sources of light which 
are to be compared. At M and M’ are two mirrors. Next in the 
train is the essential part of the instrument, namely, the Lummer- 
Brodhun photometer prism or cube. This consists of two right- 
angle prisms. The prism GFH has the outer portions of its 
hypothenuse face ground away, leaving only a small circular por- 
tion ED in the center plane. These prisms are clamped in a metal 
holder, by which they are pressed together so tightly that all air 
is excluded from between the surfaces limited by ED. Conse- 
quently, over this small circular area the two prisms become optic- 
ally homogeneous, and light will pass through one to the other 





Fig. 13.—Lummer-Brodhun Photometer. 


without diminution. The lens L is placed as a magnifying glass 
for viewing the hypothenuse surfaces of the prisms. The action 
is as follows: 

Light diffusely reflected from the left-hand side of § is further 
reflected by M and enters the surface GF of the left-hand prism. 
Those portions of this beam of light which fall upon the surface 
ED are transmitted thereby and proceed through the second prism 
to the lens L, and so to the eye. Those portions, however, which 
reach the surfaces FE and DH are totally reflected and do not 
pass into the second prism. Similarly, the light from the right- 
hand side of S falling upon M”’ is thereby reflected into the right- 
hand prism. Those portions of this light which fall upon ED 


THE MEASUREMENT OF LIGHT 439 


are transmitted and do not reach the lens L, whereas the light 
falling upon the surface FEDH is totally reflected and is directed 
toward L and toward the eye. The eye, therefore, when accom- 
modated upon the surface FH sees a circle ED, apparently of the 
same brightness as the left-hand side of 8, and surrounding that 
circle and fitting it exactly, a ring of the same brightness as the 
right side of 8. 

When the sides of S are equally illuminated, the field of the 
photometer shows everywhere the same brightness, and the line 
of separation between the ring and the disc disappears, provided 
that the lights are of the same color. If the lights are of different 





Fic. 14.—Lummer-Brodhun Photometer. 


colors, the two portions of the field will always be distinct, but at 
the point of equality can still be determined with a certain degree 
of accuracy, although settings made under these conditions are 
much more difficult to make and are far less accurate than when 
the colors are alike. 

The Lummer-Brodhun prism is used in many forms of photo- 
metric apparatus. The standard form for use on the bar photome- 
ter is illustrated in Fig. 14, in which the various portions of the 
apparatus are clearly visible. The box is made reversible so that 
inequalities of the two sides can be cancelled out. The equation 
for computing the candle-power is the same as in the case of the 
Bunsen contrast photometer. 


440 ILLUMINATING HNGINEERING 


The photometer box is obtainable also equipped with a divided 
circle, whereby it can be set to any required angle with the vertical, 
an arrangement which is convenient for some classes of work. 
It may also be had with an extra prism, whereby the sight tube 
of the photometer is brought out perpendicular to the photometer 
track. In Kriiss’s construction the perpendicular sight tube is 
axial with the box. In the Schmidt & Haensch construction it 
is parallel with the axis of the box, but slightly to one side of the 
same. Whether these constructions are preferable or not is largely 
a matter of personal opinion. 





“Fig. 15; 


Lummer-Brodhun Contrast Prism. The contrast prism is il- 
lustrated in Figs. 15 and 16. The portions r, and r, of the prism 
ABD (Fig..15) are etched, the portion r, being cut in the form 
of r, in Fig. 16, and the form r, as shown at r, in Fig. 16. GB 
and MC represent pieces of plane-parallel clear glass which are 
affixed to the prisms, as shown, and which serve the purpose of pro- 
ducing a diminution of about 8 per cent in the intensity of the 
light which traverses them. The result is that the portion of the 
field 1, is illuminated from the left-hand side of the screen to the 
full value, while interposed in the field 1, is the smaller field r,, 


THE MEASUREMENT OF LIGHT 441] 


which is illuminated from the right side of the screen to a slightly 
diminished value, the diminution being caused by the absorption 
of the glass MC. Similarly, the portions of the field r, are il- 
luminated from the right side of the screen to the full value, and 
interposed in this field is the smaller field 1, illuminated from the 
left side of the screen to. the diminished value. When the illumi- 
nations on both sides of the screen are equal, the brightness of the 
field 1, is the same as the brightness of the field r;,, and the field r, 
makes the same contrast with the field 1, that the field 1, does with 
the field r,. Hence, the photometer may be used by noting the dis- 
appearance of the vertical lines between 1, and r,, in which 'case it 
is an equality photometer, or it may be used by making the con- 





Fic. 16.: 


trast between r, and 1, the same as 1, and r,, in which case it is a 
contrast photometer. In other makes the smaller fields r, and 1, 
are given in a different form from that herewith shown. 
Sensitiveness. Since the eye is able to perceive a smaller degree 
of difference in contrast than difference in brightness, the contrast 
form of the photometer is more sensitive than the disappearance 
form. As a disappearance photometer the Lummer-Brodhun ful- 
fils the requirements that each portion of the field must be illumi- 
nated solely by the light from one source, and that there shall be 
no mixing of the illumination. With the Bunsen disc the bright- 
ness of the translucent part of the disc is.due partly to light re- 
flected from one source and partly to light transmitted from the 
other source, hence it follows that if the photometer is slightly 


442 ILLUMINATING HNGINEERING 


displaced from the position where a true setting would be made, 
the contrast between the two portions of the field is less with the 
Bunsen than with the Lummer-Brodhun screen under the same 
conditions. On this account the Lummer-Brodhun is more sensi- 
tive than the Bunsen as a disappearance photometer. Used as a 
contrast photometer the Lummer-Brodhun shows the same theo- 
retical superiority over the Bunsen screen, since each portion of 
the field is illuminated by light proper to one source alone. It 





Fie. 17.—Marten’s Photometer. 


should not be concluded from this, however, that in all cases the 
Lummer-Brodhun is more sensitive than the Bunsen. Observers 
with the Bunsen photometer become, as a result of long practice, 
very expert in its use, and are capable of getting surprisingly close 
results. Moreover, the Bunsen photometer, especially with the 
star disc, is considerably easier to use with a difference in color 
between the lights than is the Lummer-Brodhun. 

The Martens Photometer. A view of this photometer, as con- 
structed by Franz Schmidt & Haensch, is shown in Fig. 1%. A 
diffusely illuminating screen sends light by way of mirrors and 


THE MEASUREMENT OF LIGHT 443 


reflecting prisms to a bi-prism, which is fastened to one face of a 
plano-convex lens. In looking at the surface of this prism through 
the other lenses of the arrangement, one side 2 is seen illuminated 
by the light from the right-hand side of the screen, and the other 
surface by light from the left-hand side of the screen. Upon 
bringing the brightness of the two sides of the screen to equality, 
the line of division between the sides of the bi-prism disappears. 
This, therefore, makes a form of disappearance photometer which 
belongs optically to the same class as the Lummer-Brodhun, and 
which is somewhat less cumbersome as to size and weight, but 
which on account of the narrow aperture of the ocular diaphragm 
is somewhat less convenient to use. This photometer is also made 
as a contrast photometer. 


Practical Apparatus te 


The Precision Bar Photometer. For precise work, a bar pho- 
tometer, suitably equipped, constitutes unquestionably the best 
form of apparatus. As ordinarily used, the lamps to be compared 
are set up at a given distance apart at the ends of a bar or track 
on which moves a carriage carrying the photometer head. The 
bar should be straight and level and free from flexure. The car- 
riage or carriages which ride upon it should move with perfect 
freedom, but at the same time without any side play. In other 
_ words, the construction should be such that in moving from one 
end of the bar to the other the center of the photometer disc should 
at all times lie in the photometric axis, and the disc itself should 
at all times be at right angles to the photometric axis. By photo- 
metric axis is meant the line joining the lamps and lying parallel 
with the bar. 

The lamps to be measured may be supported either by carriages 
rolling on the bar or on fixed supports at a given distance from 
the end of the actual track. The supports for the lamps should 
have a vertical adjustment so that the axis of the lamps can be 
brought to the proper height, and the carriages should also have a 
vertical adjustment to bring the objects which they carry into the 
same horizontal line. Clamps should be provided whereby the 
carriages can be securely locked to the track at any required point. 

Screening. A most important consideration in the arrangement 
of any photometer is proper screening of the light. It is impera- 
tive that no stray light should reach the photometer disc, and that 


444 ILLUMINATING ENGINEERING 


its illumination should be due solely to the light radiated directly 
by the lamps alone. Hence, the background of the lamp is im- 
portant. Sometimes a black-velvet surface is placed behind the 
lamps to serve as a background. This is very efficient as long as 
it is clean, but black velvet readily collects dust and may, when 
sufficiently coated, reflect an amount of light which should be 
taken into consideration. If the ends of the room in which the 
photometer is placed are painted black, and are sufficiently distant 
from the lamps so that the illumination which one receive is very 
feeble, no other background is necessary. 

Between the lamps and the photometer head should be placed 
a series of black screens. These screens should have apertures of 
varying sizes, the largest openings being in the screens nearest the 
lamps, and the smallest ones in the screens near the photometer 
head, and very little larger than the openings in the photometer 
box. These screens should be sufficiently numerous, and should 
be so spaced that the eye, when placed at the position of the 
photometer disc and looking along the bar, can see nothing but 
the surface of the screens and the background at the end of the 
bar. The walls of the room should be entirely hidden within the 
angle at which it is possible for light to enter the photometer 
box and reach the photometer screen. The dimensions should be 
so arranged that the light from the lamp at one end of the bar 
cannot illuminate any of the screens on the opposite ends of the 
bar so as to reflect ight on to the photometer disc. When all 
these’ precautions are taken, the screens will be sufficiently black 
if they are painted with a good dull, black paint, and all danger 
of error due to stray light on the photometer disc is avoided. It 
is important, however, to arrange the screens so that the observer 
at the photometer shall not receive any of the light of the lamps 
in his eyes. It is desirable in the interests of sensitiveness, ac- 
curacy in observation and of the avoidance of stray light that the 
walls of the photometer room should be painted dull black. It is 
particularly important to avoid placing in the beam of light any 
surfaces which may receive light at large angles of inadence and 
reflect: it on to the photometer disc. 

Some of the screens may advantageously be attached to a frame 
carried by the photometer carriage. If three or four screens are 
placed on each side of the photometer carriage in this way, very 
effective screening results. The screens should be made of thin 


THE MEASUREMENT OF LIGHT 445 


material, so as to avoid the error mentioned above of reflection at 
sharp angles, or if they are not of thin material the material 
around the apertures should be chamfered off to a thin edge which 
can reflect no light. Many photometrists have fallen into serious 
error due to neglect of the simple precautions ae oe screening 
which are indicated above. 

Scales. ‘The scale on the photometer bar should ob preferably 
made with white lines and figures on a black background, and a 
convenient: arrangement should be provided for illuminating it 
when it is to be read. ‘The division of the scale may be either 





Fig. 18.—Bureau of Standards Photometer Bar. 


equal part, proportional or direct reading. An equal-part scale 
divides the total distance between the lights into a certain number 
of equal divisions. The length of the divisions may be purely 
arbitrary, but it is convenient to have the total number of divisions 
1000. The results of the photometric measurement must be com- 
puted according to the inverse square law. 

The proportional-part scale is divided to give the ratio of the 
candle-power of the lamps at the two ends of the bar. Unity comes 
at the center, and the readings of a scale of this sort must be 
multiplied by the candle-power of the standard in order to give 
the candle-power of the unknown lamp. ~ 


446 ILLUMINATING HNGINEERING 


A. direct-reading scale is one which reads the candle-power di- 
rectly. It is a proportional scale in which the proportionality 
~ numbers are already multiplied by the candle-power of the standard 
with which the bar is intended to be used. While a direct-reading 
scale on an ordinary photometer reads candle-power, on an il- 
lumination photometer it may be calibrated to read directly in 
units of illumination. 

The most commonly used type of precision photometer bar is 
the Reichsanstalt type, in which the tracks are of circular cross- 
section, either of steel tubing or of cold-rolled shafting, about 1 





Fia. 19.—E. T. L. Standard Photometer Room. 


inch in diameter, and the carriages run on three wheels, two of 
which are spool-shaped so that they fit perfectly on the track at 
all points without side play. , 

As an illustration of precision nhokaate apparatus, the pho-— 
tometer bar with its auxiliaries used at the Bureau of Standards 
in Washington, and shown in Fig. 18, may be cited. | 

The bar is of the Reichsanstalt pattern, 3 meters in length, and 
is equipped with a Lummer-Brodhun contrast photometer. Numer- 
ous screens are provided, as seen, so that stray light is effectually 
excluded from the photometer disc. The lamp to be measured is 
enclosed in a cylindrical screen and is affixed to a carriage which 


THE MEASUREMENT oF LIGHT 44% 


is seen near the left-hand end of the bar. The comparison lamp, 
at the right-hand end of the bar is also supported on a carriage 
which is connected by a rigid rod to the carriage on which the 
photometer head is placed. A rotating sector disc is also seen on 
the photometer bar. This disc can be placed on either side of the 
photometer head, and is used for cutting down the intensity of 
either light in a fixed ratio. The measurements of voltage and 
current are made by means of a potentiometer, the galvanometer 
of which is seen attached to the wall, so that the spot is thrown 
on a translucent screen directly in front of the photometer. 

A further illustration is furnished by the standard photometers 
of the Electrical Testing Laboratories. ‘Two such photometers 
are placed back to back in a room about 12 feet by 30 feet (Fig. 





Fie. 20.—E. T. L. Standard Photometer Supports. 


19), with a high ceiling, which is set apart for standard photome- 
ter work, and the walls of which are painted black. The photome- 
ters, while similar in many particulars, differ in certain details. 
Both bars are of the Reichsanstalt pattern, but that of photometer 
B has a modified form of supports, shown in Fig. 20. The car- 
riages also have been designed with particular care to make them 
mechanically perfect. To minimize the effect of slight irregu- 
larities in the track in tilting the photometer head, and thus dis- 
placing its center line from a position directly over the index 
mark, the distance between the horizontal wheels of the carriages 
of this bar has been made greater than usual. A set of screens 


448 ILLUMINATING ENGINEERING 


is attached to the photometer carriage and move with it. Other 
screens are placed on the bar as required. ‘This bar is especially 
intended for use with the working standard lamp at a fixed dis- 
tance from the photometer disc. This distance is fixed by rods 
and can be adjusted as required, so as to bring the illumination 
on the working standard side of the photometer to a given and 
definite value, a procedure which is useful in order to make the 
photometer direct reading. In photometer A, which is used ordi- 
narily: with the moving photometer head and a fixed distance be- 
tween lights, the actual lamps L and L’ are placed on small sepa- 
rate tables and not on the bar. These tables are bolted securely 
to the floor in such:positions that the lamps come exactly 5 meters 
from each other. The bar-being graduated over a range of 3 
meters, each lamp is placed 1 meter distant from the end of the 
graduations. ‘The true position of these lamps is indicated by a 
pair of plumb-bobs at each end of the bar, the plane of the plumb- 
lines being at right angles to the bar. With this arrangement 
the entire length of the photometer track is available for the move- 
ment of the photometer head. 

Several tables for lamps are provided. One of these is equipped 
with a universal lamp rotator; another with the mirror form of 
universal lamp rotator; a tag has a simple rotator with four 
mercury cups for nontace and a fourth is designed to carry a 
10-candle-power pentane standard lamp, or some other source of 
light as required. One table with its equipment can be removed 
from its position at the end of the bar and another placed in its 
stead, and bolted to the floor. at eee the proper distance with 
very little trouble. 

The dimensions of the room are such that a greater distance 
between lights than 5 meters can be had if desired by simply 
moving the lamp tables farther apart. Where a shorter effective 
photometer bar is required the lamp can be mounted on a moving 
carriage on the track, :as is done at the Bureau of Standards. This 
arrangement, then, has the advantage of very great flexibility, 
which is obtained at no expense of accuracy. Two-scales are at- 
tached to the photometer bar. One of these is divided into half 
centimeters, thus making a thousand-part scale when the lamps 
are 5 meters apart. The other is a proportional scale, correspond- 
ing to this distance, having unity at the center of the bar. Pho- 
tometer B has also two scales, the proportional scale being grad- 


THE MEASUREMENT OF LIGHT 449 


uated for a fixed distance between the photometer head and the 
working standard. 

The essential details of the electrical measuring arrangements of 
this photometer room are as follows: 

Against the wall of the room is placed a wooden table carrying 
switches and rheostats, and all of the electrical measuring appa- 
ratus. At one end of the table, near the right-hand end of the 
bar, is a potentiometer P and equipment for the measurement of 
the voltage on an incandescent lamp. Near the other end of the 
table, which comes near the photometrist’s position on the bar, 
is another potentiometer P’ for measuring the current. Simul- 
taneous measurements of voltage and current. can be made by two 
observers. The observer who measures the voltage has also to put 
lamps in position on the photometer. The other observer who 
measures current is the one who actually makes the photometric 
settings. The galvanometers are overhead and throw the spot 
of light on to scales over the potentiometer table. The arrange- 
ment is designed with a view to minimizing errors in electrical 
measurement due to current leaks, etc. The voltage on either 
lamp can be controlled either from the méasuring table or by the 
photometrist standing at the photometer bar. | 


Industrial Photometers. 


Photometers for Incandescent Lamps. The type of photometer 
used by the Electrical Testing Laboratories in industrial work is 
illustrated’ schematically in Fig. 21, which may be taken as repre- 
sentative of its class. The apparatus is supported-on a table about 
9 feet in length and 3 feet high. The distance between lights is 
100 inches (about 2.5 meters). The central portion of the table 
is occupied by a box, open in front, which contains the photometer 
track T, the photometer sight box P, and -the reading scale 8. 
It is intended that the photometer shall be used by two operators, 
one of whom makes the photometer settings and reads the indi- 
cations of an ammeter connected in the lamp circuit, and the other 
of whom manipulates the lamps which are being measured and 
reads the voltmeter. The scale § is read by the photometer setter. 
As a check on the readings by this operator the scale 8S’, graduated 
similar to 8S, may be read by the other operator. This scale is 
etched or painted on a steel tape attached to the sight box and 
running over the pulley A. The tape is kept taut by a spiral 


ILLUMINATING ENGINEERING 


450 











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LOSE ANN, , Nl 















THE MEASUREMENT OF LIGHT 451 


spring and is read from the pointer B. In order that the operator 
may move the sight box with a minimum of inconvenience, cords 
run from it over pulleys which can be manipulated by an endless 
belt C, passing along the entire front of the photometer. The 
comparison, or working standard lamp, is placed on its support 
at L’ and is boxed in. The primary standard lamp, or the lamp 
to be tested, is placed on the rotator at L. The rotator is driven 
by means of a belt through a clutch operated by a treadle on the 
floor. By means of the clutch, the rotator can be thrown out or 
the direction of its rotation can be reversed. The reversal feature 
facilitates screwing lamps in and out of the socket. Current is 
carried to the rotator socket by means of mercury cups M and M’. 
Potential leads are also brought from these mercury cups or from 
nearby positions on heavy wires leading to the mercury cups. These 
connections need to be made with very great care, so that the volt- 
age indicated by the voltmeter will be as closely as possible the 
same as the voltage actually applied to the leads of the lamp. 
In series with the lamp L are placed the two slide-wire rheostats 
R and R’, the one being intended for the use of the operator who 
handles the lamps and the other for the photometrist. A quick- 
throw switch is introduced into the lamp circuit, by means of 
which L is thrown out of the circuit and another similar lamp 
is substituted for it, so that the load upon the storage battery 
is kept practically constant, and interference of photometers with 
each other due to voltage fluctuations is avoided. The voltmeter 
wiring may be so arranged that the voltage on either the test lamp or 
the substitute lamp is always impressed on the voltmeter, whereby 
the needle of the voltmeter is not allowed to return violently to zero 
on removing the test lamp, and is used always heated to its maxi- 
mum temperature. The slide-wire rheostat R’ is in series with the 
lamp L’. The voltmeter, which is of the large or laboratory stand- 
ard type, is enclosed in the drawer which is seen at the left-hand 
end of the photometer and is seen through a cover of plate glass. 
Potential leads are brought from between L and L’ to a quick-throw ~ 
switch placed near the voltmeter, by means of which the voltage on 
either lamp can be read. Particular attention is paid to screening 
the photometer disc from stray hight. A series of screens E are 
supported by a lazy-tongs arrangement attached to the photometer 
box and moving with it, whereby the screens are kept equally spaced 
and the entrance of stray light is effectually prevented. All parts of 


452 ILLUMINATING ENGINEERING 


the apparatus are painted dull black. The scale of the photometer 
is direct reading in candle-power, the 16-candle-power point at the 
center of the scale. 

For central station, or for lamp-factory use, a number of the 
refinements which have been introduced into the above photometer 
may be omitted and the apparatus may in this way be considerably 
simplified. The operation of the photometer is as follows: 

First, several standardized incandescent lamps are used. One of 
these lamps is placed in the rotator at L and brought to its stand- 
ard voltage. Supposing this to be a 16-candle-power lamp, the 
sight box is set to read 16 candle-power. Then, by adjusting the 
rheostat R”’, the lamp L’ is brought to the point where its light 
just balances the light from the standard 16-candle-power lamp 
and the voltage on L’/ is noted. By a repetition of this operation 
and by checking with other standard lamps, it is possible to adjust 
L’ with great accuracy, so that the photometer is direct reading. 
The noted voltage on L’ is then maintained during half a day, pro- 
vided there is no change of photometer operators during this time. 
On account of the individual peculiarities of different operators the 
voltage required on L’ to make the photometer direct reading varies 
from operator to operator. Hence, the operation of setting the 
working standard must be repeated whenever operators are changed. 
The substitution method used eliminates errors from the results 
obtained on the test lamps due to individual peculiarities and to lack 
of symmetry of the apparatus. 

Ammeter Corrections. The voltmeter terminals may be so placed 
that the ammeter in the circuit will or will not record the current 
which passes through the voltmeter. In case this current is re- 
corded, ammeter readings must be corrected for it. The ammeter 
can best be calibrated from time to time by the use of a standard 
ampere lamp, a lamp, the current consumption of which has 
been carefully determined at a given voltage or series of voltages. 
By using such a lamp the necessity for making a separate cor- 
rection to the ammeter for voltmeter current may be avoided, the 
latter appearing merely as a part of the error of the instrument. 

Wiring of the Photometer. There are two plans for wiring the 
bar for the photometry of incandescent lamps. For accurate work, 
where a storage battery furnishes a steady voltage, the system of 
separate circuits should be employed. This is shown in Fig. 22. 
An adjustable resistance is placed in series with each lamp and the 


THe MEASUREMENT OF LIGHT 453 


potential is adjusted separately and independently to the required 
value. A voltmeter connected through a throw-over switch serves 
to measure the potential on either lamp. It is even better to 
supply the two lamps from separate batteries. 





Fig. 22.—Diagram of Separate Circuit Wiring. 





Fig. 23.—Diagram of Similar Circuit Wiring. 


When the voltage is not steady, the system of “same-circuit ” 
or “similar-circuit ” wiring should be adopted. In it the lamps 
are placed in multiple with each other, and in series with a rheo- 
stat R as shown in Fig. 23. A slider rheostat R’ is placed in series 
with the working standard lamp L’. This should be wound with 


454 ILLUMINATING: ENGINEERING 


wire of low-temperature coefficient. The operation is as follows: 
A reference standard lamp is placed at L and is brought: approxi- 
mately to its proper voltage by manipulating R.. The working 
standard is then brought to its right candle-power by adjusting R’ 
until the photometer dise shows a balance. A reference standard 
of different voltage is placed at L and without disturbing R, R’ 
is changed until the photometer is again balanced. This gives 
two positions of the slider R corresponding to lamps of two. differ- 
ent voltages at L. The slider may from these two positions be 
graduated so that the proper voltage can be impressed on L’ for 
a lamp of any voltage at L simply by moving the slider +o the 
proper position: asindicated by.this. graduation. «After this. adjust- 
ment.has. been made, the lamp to be measured is placed at. L and 
L/ is brought to the proper voltage: by a»movement of the~slider 
to the mark corresponding to the voltage at which L is to be 
measured. If the voltage on the line variés, the voltage on both 
lamps suffers proportional variations, and the accuracy of the 
measurements is not’ seriously impaired: *-In this way lamp tests 
can be made without the use of a voltmeter. The wiring can 
easily be so arranged that it can be changed from separate circuit 
to same circuit by throwing a switch or two. , ; 

The * Sliding- Scale” Photometer. This form of photometer is 
much used in factory practice in rating incandescent lamps for 
voltage. It is wired according to the same circuit plan, and the 
voltmeter is dispensed with entirely. The scale of the photometer 
is graduated to indicate Soe ates of candle-powers. The 
principle is as follows: 

Suppose that a lot of 16- candied -power lamps of voltages ranging 
from 112 to 120 are under test. “Lhe working standard lamp on 
the photometer is set by putting a 116- volt, 16-candle-power stand- 
ard lamp in the test end of the photometer and adjusting the rheo- 
stats until the photometer setting comes in the middle of the bar. 
The voltage on the standard lamp during this operation need not 
be exactly 116, but should not vary too widely therefrom. When 
this adjustment has been made, any other similar 116-volt lamp 
will give a setting in the middle of the bar, but a lamp intended 
for a lower voltage will give what is, from the candle-power point 
of view, a higher setting and vice versa. If, now, the relation 
between the voltage variation and candle-power variation of such 
a lamp is known, the bar may be divided so as to indicate voltages 


THE MEASUREMENT OF LIGHT 455 


instead of candle-powers. Or, the bar may be graduated em- 
pirically by photometering a series of lamps already rated for 
voltage by regular photometric methods. This style of photometer 
is adapted to the very rapid rating of lamps to a degree of accuracy ~ 
sufficiently great for commercial purposes. It labors under one 
great defect, which arises from the fact that lamp filaments with 
different degrees of treatment do not have the same voltage-candle- 
power relation. Jn consequence of this it is necessary either to 
have different scales for different types of filament, or to use a 
given scale only within narrow limits of voltage. 





Fig. 24.—Plan of Differential Wiring. 


Differential Wiring. Another system of wiring which is par- 
ticularly applicable to photometers employed in inspecting lamps 
for accuracy of voltage rating is the “ differential” system 1il- 
lustrated in Fig. 24. In this system a low-reading voltmeter is 
used to indicate the difference between the voltage on the working 
standard lamp and the test lamp. In the preliminary setting of 
the photometer the rheostats are so adjusted that the photometer 
setting comes at the mid-point of the bar when a standard lamp 
is in the test-lamp end of the photometer, and the voltmeter indi- 
cates the difference between the true-rated voltage of the standard 
lamp and the basic voltage chosen for the photometer. The latter 

18 


f 


6 


ILLUMINATING ENGINEERING 








Fic. 25.—Letheby Photometer. 


THE MEASUREMENT OF LIGHT 45% 


may be made 100 volts when testing lamps of the 100 to 120-volt 
type. In this case the true voltage of a test lamp is the voltage 
indicated by the voltmeter plus 100 volts. If the pressure in the 
supply circuit is variable it leads to more reliable results to use 
a voltmeter having its zero in the center of its scale, and to add 
or subtract its indications, taking the basic voltage at some mean 
value, as 110 volts.* 

Photometers for Gas Testing. A well-known form of photometer 
for determining the illuminating power of gas is the Letheby 
(Fig. 25). This is a simple bar photometer, using a Bunsen or 
Leeson dise. The bar is a vertical board of seasoned wood, attached 
to a rail which forms the base of the board. The photometer car- 
riage straddles the board and runs on wheels which rest on the 
rail. The scale is marked on the board and is graduated so as 
to be direct reading in terms of the standard habitually employed. 
The preferred distance between lights, according to the American 
Gas Institute, is 60 inches. The proper location of the lamps is 
indicated by plumb-bobs, while the rest of the table is equipped 
with the necessary gas-measuring and controlling apparatus, such 
as precision meter, pressure governor and pressure gauge. 

In England, the London Gas Referees have preferred the Har- 
court table photometer, which in principle is a modified Foucault 
photometer, and in which the photometric setting is made by vary- 
ing the flow of gas which is being tested until a balance is es- 
tablished. 


LeotureE II 
Portable and Illumination Photometers 


Weber Photometer.; ‘This may be taken as the prototype of 
most of the illumination photometers for precision work which are 
in use at the present time. A view of the instrument, as a whole, 
is shown in Fig. 26, and the same is shown again in cross-section 
in Fig. 27. The tubes A and B are affixed to each other in such 
a way that B can rotate about the axis of A and can be fixed at 
any required angle with the vertical. In the lantern d is a small 
benzine lamp. This is equipped with a flame measure at q so 


* See Marshall’s article in the Transactions of the American Insti- 
tute of Electrical Engineers, Vol. XX, p. 77. 
+ See Weber, Journal fiir Gasbeleuchtung, 1898. 


458 ILLUMINATING ENGINEERING 


that the height of the flame can be adjusted to just 20 millimeters 
by turning the knob T. Within the tube A is a milk-glass plate f, 
which can be moved back and forth along the axis of the tube by 
means of the rack and pinion v, and the distance of which from 
the flame can be read on the millimeter scale r. The illumination 
of the plate f is inversely proportional to this distance r. At the 





































































































































































































Fic. 26.--Weber Photometer, View. 


intersection of the axis of the tubes A and B, is placed a Lummer- 
Brodhun prism p, whereby the observer in looking in at O sees 
the illuminated plate f, and whatever else is in the prolongation 
of the tube B. If, for instance, the tube B is pointed toward the 
illuminated surface of a white card, this white surface is seen 
contiguous to the surface of the screen f, and by moving the screen 
f the brightness of f and the brightness of the card can be made 
equal. From the reading r, using a previous calibration of the 
instrument, the illumination of the card can be determined. If 
the distance between the lamp and the card which it illuminates 
is known, the candle-power of the lamp can be computed by divid- 
ing the illumination by the square of the distance. ‘The instru- 
ment, however, is equipped with other means for measuring candle- 
power. The box g near the end of the tube B is arranged to 


THE MEASUREMENT OF LIGHT 459 


receive one or more of a series of milk-glass plates furnished with 
the instrument. If the tube B is pointed toward the lamp, and the 
illumination on the plate in g is measured by f, the candle-power 
of the lamp can be determined. Here, again, a previous calibra- 
tion of the instrument is required. 

There is also furnished with the instrument a strongly diffusing 
plate, which can be fitted to the end of B in place of the tube k. 
This plate can be used instead of the card for measuring general 
illumination. When this plate is used, the instrument requires 
still another constant of calibration. 





Fic. 27.—Weber Photometer, Section. 


The Weber photometer has done excellent service for many 
years. Its chief disadvantage is the benzine flame which, however, 
ean readily be replaced by a miniature tungsten lamp held at 
constant current. With the benzine flame, a variation of 1 muilli- 
meter in height corresponds to an error of approximately 5 per 
cent, so that this adjustment, which is not an easy one to make 
in any case, must be carried out with great precision. ‘Trouble is 
sometimes experienced with this photometer, due to light leaking 
about the screen f or to diffuse reflection from the interior of 
the tube A. 

Furthermore, when the milk-glass plate is near the clear glass 
which protects the benzine flame, multiple reflections between 
these plates may cause a departure from the inverse square law. 
These difficulties may be overcome by calibrating the scale. While 


460 ILLUMINATING ENGINEERING 


more convenient instruments have been produced in recent years, 
it cannot be said that any is applicable to a greater diversity of uses 
than the Weber. 

Sharp-Millar Photometer.* This instrument is designed par- 
ticularly with a view to a wide range of usefulness, to convenience 





Fic. 28.—Sharp-Millar Photometer. 


in handling and accuracy in results. A view of it is shown in 
Fig. 28, and a plan and elevation in Fig. 29. The body of the 
photometer is a hard-wood box about 2 feet (61 cm.) in length, hav- 
ing a hinged cover so that the entire interior is accessible. The 
movable part is the working standard which is a miniature tungsten 








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P CiiaRLY hata | 
8 2@ ¥\ 
+ 
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Fic. 29.—Side Elevation and Plan of Photometer. Sharp-Millar 
Photometer. 


lamp. The working standard carriage is moved by means of an ex- 
ternal knob which acts on a cord and pulleys. The hght from the 
lamp shines on a translucent glass plate near the left-hand end of the 
photometer box proper. The photometric device is a modified Lum- 


* See Electrical World, January 25, 1908. 


THE MEASUREMENT OF LIGHT 461 


mer-Brodhun prism in which, by the addition of another totally re- 
flecting surface, rays from opposite directions are brought to the 
sight tube. At the extreme left end of the photometer is fixed an 
elbow tube through which the light passes to the photometer prism. 
This tube may be turned to any angle, and hence serves for the re- 
ception of light or illumination from any direction. In the elbow of 
the tube is a reversible plate, one side being a diffusely reflecting 
surface used in measurements of candle-power, and the other side a 
mirror used in connection with a diffusing translucent plate of glass 
at the end of the tube used as a test plate in measuring illumination. 





Fic. 30.—Diagram of Screen System. Sharp-Millar Photometer. 
Absorbing Screens. 


This test plate is removable. The scale of the photometer, which 
is direct reading in candle-power or foot-candles,* is on translucent 
material, and is so arranged that by turning a knob which raises 
a shutter on the inside of the box, the light from the comparison 
lamp shows the position of the pointer on the scale, a feature which 
is of much practical convenience. A system of movable screens 
is arranged inside the box, whereby no light reflected from the 
sides of the box is able to reach the diffusing glass window. In 
order to extend the range of the instrument, two absorbing screens 
are provided, as shown in Fig. 30. One of these has a higher 
absorbing coefficient than the other. They are arranged in such a 
way that by turning a knurled head either screen may be brought 
into the path of the light either from the working standard or 
from the unknown source. One of the screens transmitting, 


* Since 1 foot-candle equals 10.76 meter-candles or lux it is evident 
that the working standard lamp can be adjusted so that the scale reads 
directly in lux with a constant of 10. 


462 ILLUMINATING ENGINEERING 


roughly, 10 per cent of the light, and the other 1 per cent of the 
light, the range of the instrument is from 0.01 part of the mini- 
mum reading of the scale to 100 times the maximum scale reading. 
The instrument is used as follows: 

1. For measuring candle-power, the elbow tube is turned toward 
the lamp to be measured, and the diffusing side of the elbow plate 
is turned inward. The distance between the lamp and the elbow 
plate is adjusted equal to that given in the calibration certificate 
of the instrument for making the instrument direct reading. Or, 
the instrument may be recalibrated by reference directly to a 
standard of light. Readings of candle-power are then taken from 
the scale. 

2. Measurement of illumination. First Method. The mirror 
side of the elbow tube is turned in and the diffusing test plate 
is placed on the end of the elbow tube. This test plate is then 
brought into the plane in which the illumination is to be meas- 
ured and settings are made as before. With the proper current 
through the working standard lamp, the results are given directly 
in foot-candles of illumination. Second Method. A detached 
test plate is used. This should be a flat plate with a very perfectly 
diffusing surface. This test plate is placed in the plane in which 
the illumination measurements are to be made, the diffusing plate 
is removed from the end of the elbow tube, the mirror being left 
turned in. The photometer is then placed in a convenient posi- 
tion with the elbow tube turned toward the test plate. The posi- 
tion should be so chosen that neither the instrument nor the ob- 
server throws a shadow or disturbs the illumination conditions on 
the test plate. In looking into the eye-piece, the illuminated 
test plate is then seen directly in the field, and its brightness may 
be balanced against the brightness of the diffusing glass plate 
in the photometer... The calibration of the instrument for use 
after this plan will not be the same as with the detachable test 
plate on the elbow tube, since the reflection coefficient of the de- 
tached test plate and the transmission coefficient of the elbow- 
tube test plate will not’ be the same. The distance between the 
detached test plate and the photometer is immaterial, provided 
the test plate is so large that the field of the photometer is en- 
tirely covered. The angle at which the test plate is viewed should 
not differ too much from normal, since, if it does, the variations 
of the reflection of the test plate from the cosine law become so 


THE MEASUREMENT OF LIGHT 463 


marked as to introduce too great an error. Within proper limits, 
the magnitude of the angle makes no difference in the results. 

The photometer may also be used for the measurement of specific 
intensity, etc., provided it is first calibrated for this purpose. 

The photometer is used solely by the substitution method. It 
is calibrated by reference to known standards of candle-power or to 
known illuminations, the current through the working standard 
lamp being adjusted to such a value that the instrument is direct 
reading in the required unit. 






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Fic. 31.—Beckstein Photometer. 


Beckstein Photometer. ‘This photometer is shown in Fig. 31. 
It makes use of the Lummer-Brodhun prism and of a miniature 
incandescent lamp as a standard. The photometric settings are 
made, however, by a variable sector disc on the plan of the Brod- 
hun sector. An illumination test plate is shown at A. The tube 
carying A can be rotated to any required angle about the instru- 
ment. Candle-power measurements can also be made with the in- 
strument by rotating the shell to which A is affixed into the posi- 
tion where the shielding tube shown in the illustration comes in 
line with the aperture in G’. A motor is required to drive the 
Brodhun sector arrangement. 


a a ILLUMINATING ENGINEERING 


Blondel and Broca Photometer. ‘he photometric scheme con- 
sists of two crossed prisms B (Fig. 32), in the face of which are 
the diffusing plates G and G’. This portion of the apparatus can 
be used as an ordinary photometer on a bar. ‘To it can be at: 
tached tubes which have lenses F and F” at their extremity, and 
which are fitted with the cat’s-eye photometric diaphragms D and 
D’. At the end D may be attached a lantern containing a flame 
standard light. At the end D’ may be attached the rotating elbow 
tube carrying the mirror E’ and a test plate A. Evidently the use 
of this photometer is similar to that of others described. 

Marten’s Universal Photometer.* ‘This photometer employs the 
same bi-prism arrangement to produce the photometric fields that 


F 
SASSY 








Fria. 32.—Blondel-Broca Photometer. 


have been described in the Marten’s photometer above. ‘The present 
instrument, which is shown in Fig. 33, has a miniature incan- 
descent lamp as standard, and employs the polarizing principle 
for photometric setting. The rays of light from the standard 
lamp and from the unknown source which reach the photometer 
by way of the tube T pass through the Wollaston prism P, by 
which each beam is divided into two beams polarized in planes at 
right angles to each other. The ordinary beam from one source 
and the extraordinary beam from the other are deflected out of 
the fleld. After passing the bi-prism the beams traverse the Nicol 
prism N. If the plane of the Nicol lies at 45° to the planes of 
polarization of the beams which have passed the Wollaston prism, 
both rays are reduced in intensity by an equal amount. In turning 
the Nico] from this position, one beam is diminished in brightness 


* Verh. der Deutschen Physikalischen Gesellschaft. 1903 


THE MEASUREMENT OF LIGHT 465 


and the other increased. The position of the Nicol is read from 
a divided scale. The opaque diffusing plate F, which is used for 
candle-power measurement, may be replaced by a milk-glass plate 
for illumination measurements or may be removed entirely for 
the measurement of surface brightness. The range of the instru- 
ment is increased by the use of smoked-glass screens which are 
placed before the opening b. 


Auaiiary Apparatus 


Certain apparatus is necessary for determining the photometric 
elements of sources of light. Not only is the candle-power in a 
given direction wanted, but often candle-power in various direc- 


x ¥ 


SSSSSSSSSNG SS 


Sssou 
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Z 
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Za ca 


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Fic. 33.—Marten’s Universal Photometer. 


tions in a horizontal plane, or the mean horizontal candle-power 
is required. It is also requisite to know candle-power in various 
directions in the vertical plane passing through a lamp, and finally 
the mean spherical candle-power and the mean hemispherical 
candle-power need to be known. In order to measure these photo- 
metric elements, various mechanical devices are required. 


Mean Horizontal Candle-Power 


The mean horizontal candle-power of a source of light may be 
measured by taking its candle-power at a sufficiently large number 
of positions equally spaced about the lamp in a horizontal plane. 
For instance, in most cases, by taking 36 measurements 10° apart. 


466 ILLUMINATING ENGINEERING 


and averaging these, the mean horizontal candle-power is obtained. 
In the case of incandescent lamps a simpler method is applicable, 
which is in practically universal use in this country. This consists 
in rotating the lamp at such a speed that the impression on the 
eye viewing the photometer disc is one of constant, or nearly con- 
stant, illumination. For carbon-filament lamps this speed is taken 
at about 180 revolutions per minute. With lamps having a larger 


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Fig. 84.—4-Cup Rotator. 


number of parallel vertical portions of the filament, such as tan- 
talum and tungsten lamps, a lower speed of rotation is sufficient. 
In any case the speed of rotation should be high enough so that 
no very strong flickering is observed, and at the same time it 
should not be so high as to produce any marked deformation of the 
filament. ‘The rotators for incandescent lamps are made in large 
variety of forms, the principal requirement being that the appa- 
ratus should be sufficiently strong and rigid, and that the arrange- 
ments for carrying current to the rotating portions should be per- 


THE MEASUREMENT OF LIGHT 467 


fectly reliable. For continuous work, it is most common to use 
mercury contacts for this purpose. Where brushes are used, it is 
necessary that the potential on the lamp should be taken from the 
collector rings by an extra pair of brushes, insulated from the pair 
which bring current to the rings. Thereby any drop in voltage 
taking place at the brush contacts is not measured as part of the 
voltage on the lamp. As an extra precaution, in precise work, 
voltage may be taken from the shell of the lamp itself. For this 
purpose the socket of the lamp is split and current is led in at one 






























































Fic. 35.—Compound Rotator. 


side and voltage taken out from the other side. A rotator com- 
bining a socket of this description, with four mercury cups, is 
shown in section in Fig. 34. Evidently a similar arrangement can 
be made for lamps which, like tungsten lamps, are best burned in 
a pendant position. 

The method of the German Elektrotechnischer Verein for get- 
ting the mean horizontal candle-power of incandescent lamps con- 
sists in placing behind the lamp two mirrors, one at each side, so 
that the mirrors reflect the light of the lamp on to the photometer 
dise. These mirrors make with each other an angle of 120°. The 
line of the intersection of these two mirrors hes 9 centimeters 


468 ILLUMINATING ENGINEERING 


behind the axis of the lamp. The photometer disc is then illumi- 
nated by the light of the lamp directly and by the light from two 
other directions equally spaced in the horizontal plane received 
by reflection. This method has been used at the Electrical Testing 
Laboratories in the photometry of tungsten lamps. 

In Fig. 35 is shown a compound rotator; that is, one in which 
the lamp can be measured in all vertical angles about the lamp, and 





Fic. 36.—6-Mirror Rotator. 


the values so obtained can be used in computing the mean spherical 
candle-power. 

By means of the device * shown in Fig. 36, the mean candle- 
power in two directions is obtained in each measurement. The 
frame about the lamp carries six mirrors, so that if the frame is 
set at 10° from the vertical, the photometer receives light simul- 


* Physical Review, Vol. II, p. 181, 1900. 


THE MEASUREMENT OF LIGHT 469 


taneously from 10° from the zenith and 10° from the nadir of 
the lamp. The use of this arrangement decreases the number of 
measurements required when mean spherical candle-power is to 
be obtained. If measurements are made by the substitution method, 
the absorption of the mirrors is eliminated. 

A greatly enlarged apparatus on this plan, using, however, only 
three mirrors is in use in the Electrical Testing Laboratories for 
the measurement of the distribution about shades and reflectors, 
and is illustrated in Fig. 37. These mirrors have a dimension of 
22 by 30 inches (55 by 75 cm.). 

A design of three-mirror distribution apparatus, as modified by 
Mr. W. F. Little, is shown in Fig. 38. 


Hj -—- —-——. —. — (5B) 





l 
—-—- fot of Ligh 


A 6.c °Sirrors. 





=Floor- 


PIG 91: 


The distribution about lamps may also be obtained, using some 
scheme involving the elevation of the lamp above the plane of the 
photometer, as in Fig. 39. If the lamp is elevated to such a height 
above the photometer that at the photometer head its rays inter- 
sect the photometric axis at an angle 6, then the rays so intersecting 
the photometric axis proceed from the lamp at an angle below the 
horizon equal to @. If, now, the photometer is rotated through 
an angle 14 6, the rays from the lamp being measured, and from 
the standard lamp, will fall upon the photometer disc at equal 
angles, and photometric measurements can be made by moving 
the standard lamp. Wedding has used this arrangement in the 
photometry of are lamps, extending it by placing a photometer 
bar and photometer on each side of the lamp and making simul- 
taneous measurements in two directions. Dibdin’s radial photome- 


470 ILLUMINATING EKNGINEERING 


ter is constructed on this principle. In this photometer the dis- 
tance between the lamp and the photometer disc is kept constant 
by means of an arm, and by a suitable arrangement the photometer 
dise is tilted so as to bisect the angle between the rays coming 
from the lamp and the rays from the working standard. In using 
a photometer of this character, particular care must be taken in 
screening the disc from stray light. 





Fig. 38.—Three-Mirror Light Distribution Apparatus. 


An arrangement which has been frequently used in the photome- 
try of arc lamps is the crane illustrated in Fig. 40. This is used 
in connection with a mirror M placed at 45° to its axis, the axis 
of the mirror being located in the photometric axis. By means 
of this simple arrangement, which is due to Ayrton and Perry, the 
light from the arc lamp can be measured in any vertical angle 
except the zenith. In an arrangement used in the photometry of 
arc lamps in the engineering laboratory of the Massachusetts In- 
stitute of Technology, the photometer is placed on a long steel 


THE MEASUREMENT OF LIGHT 471 


structure which is movable about a horizontal axis. At the oppo- 
site end of this structure the arc lamp is suspended. When this 
structure is tilted through an angle 6, the rays of the lamp pro- 
ceeding at —6@ are photometered. The arrangement is very cum- 
bersome and expensive. 

A yery simple arrangement for measuring the intensity of hght 
at various vertical angles, and which is described here for the 
first time, is illustrated in Fig. 41. The long arm shown in this 
figure, which can be set at any vertical angle about the lamp, 





Fig. 39.—Elevated Lamp. 


carries at its extremity simply a diffusing white surface 8. The 
apparatus is especially intended to be used with the Sharp-Millar 
photometer, but may be used with any other suitable photometer 
with Lummer-Brodhun cube. The small mirror M at the end 
of the rotating shaft enables the observer in looking into the eye- 
piece, to see the illuminated disc at the end of the arm. He then 
compares the brightness of this disc with the brightness of the 
other side of his photometric arrangement. The distance to the 
lamp being fixed, the brightness of the disc is directly propor- 
tional to the candle-power of the lamp at the angle at which the 
19 


472 ILLUMINATING ENGINEERING 


arm is set. It will be seen that the idea of this arrangement con- 
sists in substituting for an arrangement whereby the lamp is 
moved about the photometer, or the photometer and track moved 
about the lamp, an arrangement whereby one side of the photome- 
ter disc is moved about the lamp, all of the rest of the apparatus 
being stationary. It will be seen that the size of the source of ight 
which can be measured with this arrangement is not limited by the 
size of any mirrors. 








Fig. 40.—Are Lamp Crane. 


One of the difficulties in the photometry of are lamps is the 
unsteadiness of the light due to the movement of the crater. It 
was to overcome this difficulty that Wedding employed two pho- 
tometers. Matthews * has employed for this purpose two mirrors, 
as shown in Fig. 42. These mirrors are on arms which move 
about an axis passing through the axis of the apparatus. They 
are set at corresponding angles on each side of the arc lamp. The 
illumination then on the photometer disc is proportional to the 


* Report to Committee on National Electric Light Association, 1900. 
Physical Review, Vol. 7, p. 239, 1898. 


THE MEASUREMENT oF LIGHT 473 


sum of the intensities of the arc lamp at two angles on opposite 
sides of the lamp. The rays strike the photometer disc at a con- 
siderable angle of incidence, but this difficulty and the difficulty 























Fic. 41.—Long Arm. 


due to the absorption of the mirrors is eliminated by the use of 
the substitution method. This is one of the most convenient ar- 
rangements known for the photometry of arc lamps. In these 
tests a device was employed to obviate the difficulty due to the 


474 ILLUMINATING ENGINEERING 


difference in color between the are light and the light of the incan- 
descent lamp standard, which has seldom been used in photometry. 
The plan was to use a rotating sector disc with very narrow open- 
ings so as to cut down the illumination on both sides of the pho- 
tometer disc to so low a value that color vision practically ceases. 
The results so obtained are probably not comparable with those 
obtained by ordinary methods. ‘The records of the photometer 
settings were made using a recording device consisting of a drum 
placed axially along the photometer track, and a punch on the 
photometer carriage. The punch was operated by an electro- 
magnet and made a hole in a sheet of paper on the drum as a 
record of the photometric setting. 




















SCREEN (FIXED) 


__.|_ CENTER OF PHOTOMETER 


‘han 
STANDARD LAMP (MOVABLE) ya} 0 
SE ee igs al eos 


CENTER OF ARC \ © 











Fic. 42.—Matthews Arc Photometer. 


Integration Apparatus 


Certain apparatus has been devised in recent years for the pur- 
pose of giving in one reading the mean spherical or mean hemi- 
spherical candle-power of a source of light, or the corresponding 
quantities, spherical or hemispherical, of luminous flux. This ap- 
paratus is divided into two general classes; namely, summation 
and integration apparatus. ‘The summation apparatus depends 
upon the same operation as is employed in determining any of 
these quantities by a series of measurements made at different 
vertical angles. The theory shows that the mean spherical candle- 
power of a source of light can be obtained by multiplying by a 


THE MEASUREMENT OF LIGHT AV5 


constant factor a series of terms made up of the intensity of the 
source in various vertical angles, each multiplied by the sine of 
the angle if the angles are reckoned from the vertical line, or 
by the cosine if the angles are reckoned from the horizontal line. 
In the summation apparatus individual mirrors are placed at 
various angles and so arranged that the summation of the inten- 
sities which they throw upon the photometer disc is proportional 
to the mean spherical intensity. In integration apparatus, on the 
other hand, all angles are used, and the process is a true integra- 
tion. In practice, all apparatus, whether summation or integra- 
tion, is commonly designated as “integrating apparatus.” 

Two principal types * of summation apparatus are the Matthews 
type and the Russell-Léonard type. In the first, the mirrors are 
equally spaced and the intensity of each beam of light is reduced 
in proportion to the cosine of the angle of its emission with the 
horizontal. In the second type, the mirrors are so spaced that 
they all cover zones of equal areas of the sphere surrounding the 
lamp. 


Matthews Integrating Photometer for Arc Lamps 


Matthews’ original arrangement consisted of a vertical ring of 
24 large trapezoidal mirrors, in the center of which the lamp 
was placed. The mirrors were so set with respect to each other 
that they formed a truncated pyramid of 24 sides. Viewed from 
the photometer disc, an image of the are was seen in the center 
of each mirror, so that the disc was illuminated by rays proceeding 
from the are at all angles of inclination with the vertical. 

The diminution of the intensity of the beam from each mirror, 
proportional to the cosine of the angle of inclination which it made 
with the horizontal, was accomplished by interposing between the 
mirror and the disc a sheet of glass on which a uniform layer of 
smoke of the required thickness, as shown by experiment, had 
been deposited. Inequalities in the reflecting power of the mirrors, 
were eliminated, since the absorption of the smoked glasses and 
of the mirrors was taken together, and each smoked glass was 
adjusted to its particular mirror. 


* Trans. Amer. Inst. of Elec. Eng., 19, p. 677, 1901; 20, p. 1465, 1902. 
L’Eclairage Electrique, 40, p. 128, 1904. Jour. Inst. of Elec. Eng., 32, 
p. 631, 1903. Bulletin, Bureau of Standards, I, 1905, paper by Hyde. 


476 ILLUMINATING ENGINEERING 


In the Matthews photometer in the Electrical Testing Labora- 
tories the pyramid of mirrors, each of which has the dimensions 
of 12x14x161% inches, occupies one end of a room 70 feet long. 
(See Fig. 43.) A hardwood track is laid on the floor of this room, 
extending nearly from end to end. Arranged to roll on this track 
are a stand carrying the frame containing the smoked glass sectors 
and a table on which is placed the photometer bar. The mirrors are 
hinged so that their inclination to the backboard can be varied. By 
this means they can be adjusted to focus on the photometer disc at 





Fig. 43.—Are Photometer Mirrors, Matthews. 


whatever distance the latter may be. For measuring very powerful 
lights, the photometer table and the smoked-glass sectors may be 
moved to the far end of the room and the mirrors are tilted to 
correspond, and vice versa. 

The photometer sight box is mounted rigidly in a good-sized 
box, which shields it and the eyes of the operator from stray light. 
The track employed is of the Reichsanstalt pattern. The distance 
of the comparison light from the disc is measured by means of a 
steel tape attached to the lamp carriage and passing around pulleys 
at the ends of the track. The photometric settings may also be 
recorded by the device, which also is due to Matthews, and which 
is referred to above, consisting in a roller extending longitudinally 
along the bar, on which a sheet of paper is wrapped. Attached 
to the lamp carriage is an electro-magnet which carries a punch 


THe MEASUREMENT OF LIGHT AiG 


which perforates the paper when the circuit is closed. A key 
controlling this circuit is manipulated by the photometer operator. 
By use of this a large number of settings may be recorded with 
great rapidity, a matter of great importance in dealing with a 
rapidly fluctuating source of light, such as an are lamp. After 
completing a set of measurements the readings are averaged by 
estimation, and are interpreted by use of a scale on the photome- 
ter track. 

Are lamps under test are suspended on little trolleys running 
on an overhead track. This track is in the form of a loop outside 
the photometer room, but having a switch spur extending inside 
the room through the backboard of the pyramid of mirrors. By 
means of this arrangement the lamps may be successively intro- 
duced into the photometer without interrupting their burning 
or disturbing their normal régime. 

Measurements of the vertical distribution of candle-power are 
made with the same arrangement, only the smoked sectors are 
raised out of the way, and all the mirrors excepting the two cor- 
responding to the required vertical angle are closed by blackened 
covers. ‘These measurements have subsequently to be corrected 
for the difference in the reflecting powers of the mirrors, taking 
that of the horizontal pair as unity. | 

In practice with this photometer, both in the determination of 
mean spherical candle-power and of vertical distribution, the ar- 
rangement is first standardized by making settings against a stand- 
ardized incandescent lamp of high candle-power. This lamp is 
placed with its axis horizontal in the center of the mirror system. 
By working in this way it becomes unnecessary to determine the 
distance between the lamp and the photometer disc, as well as the 
absolute value of the coefficient of absorption of the mirrors, ete. 


Matthews Integrating Photometer for Incandescent Lamps * 


A view of this photometer is given in Fig. 44. The lamp to 
be measured (which is not visible in the figure) and the photome- 
ter box are placed on opposite sides of the vertical support and in 
the axis of a narrow half-ring. This half-ring carries 11 pairs 
of mirrors with each mirror inclined at an angle of 45° to the 
ring and 90° to its mate. By this arrangement, light emitted by 


* Transactions of the American Institute of Electrical Engineers, 
1901. National Electric Light Association, 1901. 


478 ILLUMINATING ENGINEERING 


the lamp at 11 different angles in the vertical plane is caught by 
the mirrors and is reflected back along paths parallel to their 
paths of emission to the photometer disc. To diminish these 
beams proportionally to the cosine of the angle with the horizontal, 
advantage is taken of Lambert’s cosine law which declares that the 
illumination produced on a diffusely reflecting surface is propor- 





Fig. 44.—Matthews Integrating Photometer. 


tional to the cosine of the angle of incidence of the rays on that 
surface. It follows that if in the above arrangement the photo- 
metric screen is diffusely reflecting and is placed in a vertical 
position, the illumination produced by a bundle of rays leaving 
the lamp at an angle e with the horizontal, and reflected to the 
photometer disc and incident on it at the same angle, will produce 
an illumination proportional to cos e. The total illumination on 


THE MEASUREMENT OF LIGHT 479 


the disc being made up of a series of such terms covering all 
the vertical angles will be proportional to the mean spherical 
eandle-power of the source of light. 

In the above discussion it is assumed that the source of light 
is one in which the effective distribution of intensity about a 
vertical axis is uniform. In the case of the incandescent lamp this 
condition is secured by rotating the lamp as in ordinary measure- 
ments of mean horizontal candle-power. In the case of sources 
of irregular distribution which cannot be so rotated, the mean 
spherical candle-power can be found by taking the mean of a 
series of measurements made with the lamp turned so as to present 
its different aspects to the mirror system. 

It is assumed also that the mirrors are all exactly alike in re- 
flecting power, and that the photometer disc is a perfect diffuser 
of the light falling upon it, so that Lambert’s law is obeyed. 
Neither of these conditions is fulfilled in practice; hence it is 
necessary to seek some means for adjusting the apparatus to com- 
pensate for the resulting deviations. Now, the variations from 
Lambert’s law are in the sense to make the illumination produced 
by oblique rays smaller than what is called for by the law, the 
size of the variation depending upon the amount of regular or 
mitror-like reflection which the disc shows. The variations from 
Lambert’s law can therefore be compensated for by shortening the 
path which the oblique rays must traverse in passing from the 
lamp to the photometer disc. This can be accomplished by moving 
the corresponding mirror pairs radially inward toward the lamp 
and the photometer disc. The mirrors are attached to the half- 
ring by long-threaded pins, so that this adjustment is readily made 
by experiment. Variations in the reflecting power of the mirrors 
are compensated for in the same operation. 

These adjustments hold only for the particular photometer disc 
with which they have been made. Matthews has determined the 
deviations from Lambert’s law, shown by three types of surfaces. 
His curves for them are reproduced in Fig. 45. Evidently an ad- 
justment effected for a Lummer-Brodhun screen would be not at 
all suitable for an ordinary Bunsen screen. 

The position of the photometer screen is fixed and the adjust- 
ment of illuminations on it to equality may be made, either by 
moving the working standard along the horizontal bar of the pho- 
tometer or by moving a pair of mirrors. A rod is provided to which 


48) ILLUMINATING ENGINEERING 


either the working standard or the mirror pair may be attached, 
and by which they may be moved. 

This photometer, as constructed, may be used not only for mean 
spherical candle-power but for horizontal candle-power, vertical 
distribution, and for the direct determination at one measurement 
of the spherical reduction factor of incandescent lamps. 

In using this photometer the mirrors must be kept clean and the 
surface of the photometer disc must remain protected from dirt 
and anything which might change its optical properties. It is 












Percent Variation from Cosine 
ro) 


besa Sa 
Tree 

15 P 
ues ts EELS mS Eee 
(Reese eee es Me 


Fic. 45.—Lambert’s Law and Photometer Discs. 














Per cent variation from cosine relation for different screens. 
I.—Lummer-Brodhun screen 

II.—Unglazed paper. 

IlI.—Glazed paper. 


important also that the optical center of the lamp under measure- 
ment shall be exactly at the center of the half-ring. Especial care 
should be taken to exclude stray light from the photometer disc, 
since on account of the numerous mirrors present stray light is 
liable to intrude from the most unexpected directions. The ad- 
justments of this photometer are relatively difficult to make and 


to maintain. 
Blondel Lumenmeter.* ‘The source of light L (Fig. 46) is 


* Bull., Société Internationale Electriciens, Vol. IV, p. 680, 1904. 
L’Eclairage Electrique, Vol. 3, pp. 406, 538, 583, 1895. 


THe MEASUREMENT OF LIGHT 481 


placed in the center of a hollow sphere of metal S, which is care- 
fully blackened inside and out, and which is pierced with two 
vertical slits AA 18° in width, extending from pole to pole. Since 
the area of these slits is one-tenth of the surface of the sphere, 
one-tenth of the total flux from L will pass out through them. 
This flux falling on the ellipsoidal mirror MM is reflected to the 
diffusing screen G. ‘The brightness of the diffusing screen is pro- 
portional to the total flux of light of the source L, and it requires 
only a calibration of the apparatus, which may be carried out by 
substituting for L a source of known luminous flux, in order to 
determine what the proportionality constant is. 

On account of the expense of forming the ellipsoidal mirror, a 
cheaper but less exact form of the lumenmeter has been constructed 





Fic. 46.—Blondel Lumenmeter. 


in which a spherical surface of sheet metal covered with white 
paper or painted white is substituted for the mirror. This surface 
serves at once as diffusing screen and mirror. This apparatus 
cannot be used with large surfaced sources, such as are lamps 
with globes. 

The Integrating Sphere,* the use of which should be accred- 
ited to Ulbricht, consists of a hollow sphere, coated inside with a 
white, diffusing paint, and having a small window of diffusing 
glass set into it. The lamp to be measured is placed inside the 
sphere, and between it and the window is placed a white screen so 
that the direct rays of the jamp do not fall upon the window. 


* Ulbricht. Elektrotechnische Zeitschrift, Vol. 21, p. 595, 1900; Vol. 
26, p. 512, 1905; Vol. 27, p. 50, 1906. Bloch Elektrotechnische Zeit- 
schrift, Vol. 26, pp. 1047 and 1074, 1905. Corsepius Elektrotechnische 
Zeitschrift, Vol. 27, p. 468, 1906. Monasch Elektrotechnische Zeit- 
schrift, Vol. 27, pp. 669 and 695, 1906. Ulbricht & Monasch Elektro- 
technische Zeitschrift, Vol. 27, p. 803, 1906. Blondel, Bull. Soc. Int. des 
Elec., Vol. 4, p. 687, 1904. Sharp & Millar, Trans, Illuminating En- 
gineering Society, Vol. 3, p. 502, 1908. 


482 ILLUMINATING ENGINEERING 


Then the brightness of the glass window is proportional to the 
mean spherical candle-power of the lamp inside the sphere. This 
action depends upon the theorem that on the interior of a sphere 
the surface of which obeys Lambert’s cosine law the illumination 
at any point due to light reflected from all the remaining interior 
surface of the sphere is the same as at any other point in the 
interior of the sphere. In other words, the illumination on any 
portion of the interior of the sphere due to the light reflected from 
any other portion is independent of the position of the latter, and 
depends upon its area and surface brightness only. For, in a 
sphere of radius r (Fig. 47), the Ulumination at a point C produced 


Fic. 47.--Proof of Law of Sphere. 


by light reflected by another portion of the surface of the area AA 
at a distance d from the first, under the assumption that the sur- 
face obeys Lambert’s law, both for emission and for incidence, is 
ae BLA, cong 

d 

in which e is the specific intensity of the area \A considered as a 
source of hght. Substituting for d its value 2r cos 6, there is 
obtained 





The absence of any variable distance or angle in the above equation 
shows that this illumination is independent of the location of 
the area A\A. Hence each element of the surface of the sphere will 
contribute to the point C an intensity of illumination which is 
directly proportional to the illumination which it itself receives, 


THE MEASUREMENT OF LIGHT 483 


or to the flux of light which it receives. The total illumination 
at the point C will therefore be proportional to the total flux of 
light falling on the interior of the sphere; that is, to the total 
luminous flux of the lamp which the sphere encloses. 

Integrating spheres of various diameters have been constructed 
from less than 44 meter up to 5 meters. The practical form of 
a small sphere, intended for testing incandescent lamps, is shown 
in Fig. 48. This sphere is equipped with two lamp sockets on 
hinged arms, so connected that when one lamp socket is holding 
a lamp inside the sphere for measurement the other lamp socket 





Fic. 48.—Small Sphere. 


is outside the sphere, so that the lamp which has just been meas- 
ured may be removed and another one to be measured substituted 
in its place. By the motion of a single lever, these lamp sockets 
are instantly interchanged. In Fig. 49 is shown another sphere 
of two meters diameter, intended for the photometry of are lamips 
and similar large sources of light. This sphere is constructed of 
iron and is divided vertically into halves. Each half is on wheels 
running on rails, so that they may be pushed apart quite readily 
for access to the interior. The sphere contains not only the lamp 
to be measured, but also a standardized incandescent lamp by 
which the constant of the sphere is determined. A very important 


484 ILLUMINATING ENGINEERING 


consideration in connection with the sphere is the screen which 
prevents the light from the lamp under measurement from falling 
directly on the photometer window. ‘This screen should be as 
small as it may be to accomplish the result required. Both opaque 
and translucent screens are used, the translucent screen being de- 
signed to transmit enough light to the window to compensate for 
the light which it cuts off. By the proper selection of the screen 
this compensation can be made very close in all practical cases. 





Fic. 49.—2-Meter Sphere. 


In using the sphere, the substitution method in photometry must 
be employed exclusively. If the sphere is correctly designed, its 
constant may be determined by the use of a standardized incan- 
descent lamp, and the results of measurement will then be correct 
within commercial limits for other sources of light, such as are 
lamps, ete. Precaution, however, should be taken to have the lamp 
which is to be measured in the sphere at the time when the sphere 
is being standardized by the incandescent lamp, for otherwise light 
absorbed by the lamp which is to be measured, or by its parts, such, 
for instance, as the housing of the mechanism of an are lamp, or 


Tur MEASUREMENT OF LIGHT 485 


by a diffusing globe with which it may be equipped, will not be 
taken account of. In measuring are lamps it is a good precaution 
to cover as much of the housing as comes within the sphere with 
white paper. 

If the lamp to be measured is lowered half way into an opening 
in the sphere so that its light-giving center comes in the plane of 
the opening, the lower hemispherical flux will evidently be de- 
livered inside of the sphere, and the upper hemispherical flux will 
be delivered outside of the sphere. Under these conditions the 
sphere measures lower hemispherical flux or mean lower hemi- 
spherical candle-power. The principal difficulty with this pro- 
cedure is to make sure that the lamp is so placed that the flux 
will divide itself properly. 

Heterochrome Photometry 

Hitherto consideration has been given to the comparison of 
sources of light of the same, or nearly the same, color. When the 
fields of a photometer are illuminated by lights of different tints 
the eye receives not only a quantitative difference between them, 
but a qualitative difference as well, and rebels against comparing 
two things which are not of the same kind. However, within cer- 
tain limits of error, which widen as the color difference increases, 
it is possible to see when two fields illuminated with lghts of 
different tints are equally bright, and so heterochrome photometry 
is within certain limitations possible, using the simple photometer 
in any one of the forms in which it has been described above. 

The limitations and conditions which hedge in this kind of 
work should, however, be clearly borne in mind. The measure- 
ments are influenced, first, by the brightness of the photometer 
field; second, by the size of the photometer field, and third, by the 
personal equation of the observer. 

Considering the effect of the brightness of the photometer field, 
we notice that in comparing a reddish light with a bluish one, 
the reddish light is relatively brighter at high illuminations and 
relatively dimmer at low illuminations than the bluish one. This 
is the Purkinje effect. The practical result of it is that in com- 
paring an are lamp with an incandescent lamp, for instance, the 
value assigned to the candle-power of the arc lamp will be higher 
the farther away the lamp is from the disc. As the lamp is brought 
nearer to the disc, and the illumination on the disc thereby in- 


486 ILLUMINATING ENGINEERING 


creased, the are lamp apparently suffers in candle-power. Hence 
this candle-power, as measured by simple photometry, can be stated 
as a constant quantity only when coupled with it is given the 
illumination on the photometer disc at which the candle-power was 
measured. The Purkinje effect intervenes in all ordinary meas- 
urements in heterochrome photometry. It is fortunately of con- 
siderable influence on the result only when the color differences are 
very great or when the illuminations are very feeble. At such 
illuminations as are ordinarily used on photometer discs, the 
Purkinje effect is small. This effect is something, however, which 
is of importance in practical illumination. ‘The eye being rela- 
tively much more sensitive to the blue end of the spectrum at low 
illuminations, those illuminants are most efficient where low il- 
luminations are to be produced which are of bluish tint. For 
instance, in the lighting of streets where very feeble illuminations 
are of importance, an arc lamp has an advantage over an incan- 
descent lamp which is not shown by computations of the illumi- 
nations from candle-power measurements made with a brightly 
illuminated photometer disc. It is therefore of importance in the 
practical study of an illuminant in the laboratory to know some- 
thing about the value of the Purkinje effect for it, and this evalu- 
ation may be carried out by the methods of simple photometry, 
using both high and low illuminations on the photometer disc. 
From this point of view, therefore, far from being a disadvantage 
that the ordinary photometer shows the Purkinje effect this is a 
distinct advantage, since thereby the properties of an illuminant 
for the production of illumination of different degrees may be 
more definitely ascertained. If the illumination is reduced to a 
sufficient degree, the sense of color of the light disappears, and 
hence the difference in color between two illuminated fields. Under 
these conditions heterochrome photometry is practically homo- 
chrome photometry with very feeble illuminations. The illumina- 
tions are so feeble that the error of observation is probably much 
larger than would be the case if the illuminations were increased 
to the point where the color is plainly visible. Moreover, the result 
obtained under these circumstances is the result for feeble illumi- 
nation only, and may differ very considerably from the result ob- 
tained with ordinary illuminations; in other words, the Purkinje 
effect is experienced at its maximum value. Matthews has used 
this method of photometry in the measurement of are lamps, cut- 


THE MEASUREMENT OF LIGHT 487 


ting down the light from the are lamp and from the comparison 
lamp by rotating sectors of very narrow aperture. 

The second effect noted above is that when taking equally bright 
surfaces of red and blue, equally distant from the eye, the red- 
appears brighter when only a very small area of each is visible. 
If, for instance, the field of a photometer is very small, a reddish 
light is given an advantage over a bluish one. This is due to the 
fact that the image of the field falls largely on the yellow spot of 
the retina which exerts a selective absorption in favor of the redder 
light. With fields of usual dimensions the yellow-spot effect has 
httle influence. 

The third element entering into heterochrome photometry, 
namely, the personal equation, is the element which influences the 
results most largely. As has been said, the eye rebels at com, 
paring things with each other which are qualitatively different. 
There is no exact criterion for the brightness of a field of one 
color as compared with the brightness of a field of another color. 
A given observer may set up for himself a criterion with which 
he is satisfied, and by adhering to that criterion may make very 
consistent comparisons of heterochrome illuminations, but in gen- 
eral the criterion which he uses for the brightness of two fields 
will not be the same as that used by another observer. The obser- 
vations of one may be quite as consistent among themselves as are 
those of the other. Moreover, a given observer may change his 
criterion from day to day, or even from hour to hour, and so may 
not in the long run be consistent with himself. 'T'wo observers 
who have been working to different criteria may, by a study of each 
other’s photometric settings, come gradually to adopt the same 
eriterion, after which they may continue in agreement, but the 
criterion adopted by both of them may not yield results which are 
really any nearer the truth than one of the abandoned criteria 
had done. 

It would appear to be necessary, then, in any case where pre- 
cision is required in the comparison of lights of different colors; 
to take the average result given by a large number of observers. 
Since it is impracticable to use a large number of observers in 
each piece of heterochromatic photometric work, it is important 
to bridge over the gap between the color of the standard light 
used in photometry and the color of each of the other illuminants 
with which work must be done, once for all. For instance, in the 

20 


488 ILLUMINATING ENGINEERING 


photometry of tungsten lamps, it is not advisable to compare each 
individual lamp with a carbon standard of redder color value, 
but to compare the tungsten lamp with tungsten standards which 
have been standardized through a large number of comparisons by 
different observers against the carbon standard and in the stand- 
ardization of which the color gap is spanned once for all. 

In this connection Dr. Hyde has made the important suggestion 
that standards of different color values can be made as required 
by the use of standardized color screens which are interposed be- 
tween the lamp of standard color and the lamp to be photometered. 
For instance, in the photometry of arc lamps a light blue-colored 
glass would be interposed between the incandescent lamp which 
serves as a working standard and the photometer, thereby bringing 
the tint on both sides of the photometer to the same value. ‘The 
effect of this color screen will have been very carefully studied 
photometrically by a very large number of observers, and the reduc- 
tion factor introduced by interposing the color screen will have been 
determined. 

Photometers differ among themselves in their adaptability to 
heterochrome work. ‘The photometer which is best adapted to 
homochromatic work is not necessarily ‘the one which is most sat- 
isfactory for heterochrome work. A simple equality of brightness 
photometer is not in general found as sensitive as a contrast pho- 
tometer and a photometer in which there is no mingling of the 
lights in the two fields is not so easy to use as one in which there 
is a certain degree of color mixing. Of all the simple photometers 
ordinarily used the Leeson disc seems to be most satisfactory for 
work where color differences are present. ‘Through the trans- 
lucency of the paper the lights are somewhat mingled, and con- 
sequently the color contrasts do not appear so strong. 

Visual Acuity 

Another method used in the evaluation of lights of different 
color is the so-called visual-acuity method, referred to in the be- 
ginning of the first lecture. This method depends upon the fact 
that with the eye in a normal condition the limit of. field bright- 
ness with which the eye is able to distinguish objects in the field 
is a fairly sharp one. For instance, having a printed page with 
letters of a certain size on white paper, the eye is just able to dis- 
tinguish the letters with a given degree of illumination. With 


THrE MEASUREMENT OF LIGHT 489 


larger characters, a lower degree of illumination suffices, while 
with smaller ones a higher degree of illumination is required. 
Taking, then, a white card with black letters printed on it, not 
in regular words and sentences, but in a jumble which cannot be 
readily memorized, letters of different sizes being used in the make- 
up of the card, the observer can, by varying the distance between 
himself and the light, find a point at which one size of letters can 
be distinguished and the next smaller size cannot be distinguished. 
He can afterwards turn to his standard source of light and de- 
termine the corresponding distance for it. From the ratio of the 
distances the intensity of the unknown source is determined. This 





Fie. 50.—Reading Distance Instrument. 


method has been employed by various experimenters, and has given 
rise to instruments involving the visual-acuity principle. Among 
these may be noted the illuminometer of Houston & Kennelly and 
the reading distance illuminometer which has been described by 
Ryan and Steinmetz. The latter consists of a box (Fig. 50) con- 
taining the printed card and having one aperture for the admis- 
sion of the light, and another for the use of the observer. It has 
been stated that the results obtainable with an instrument of this 
character show an error of between 5 and 10 per cent. The state- 
ment has been made that the reading-distance method is the only 
true method for measuring lights of different color. This state- 


490 ILLUMINATING ENGINEERING 


ment should, however, be taken with certain reservations. It is 
evident that reading-distance measurements are made only at very 
feeble illuminations, near the lower linrit, that is, of useful il- 
luminations. This brings them within the region where the Pur- 
kinje effect is very marked and detracts very greatly from their 
applicability to the illuminants used for the production of more 
intense illuminations. Evidently in the application of the method 
it is necessary to guard against changes of the observer’s eyes due 
to fatigue, etc., between the time of the reading of the unknown 
source of light and the reading of the standard lamp. 





Fig. 51.—Whitman Flicker Photometer. 


Flicker Photometers 


Rood * discovered that when two differently colored surfaces are 
alternately presented to view in rapid succession, the sensation of 
color disappears, or that the color sensations of the two are mingled, 
although a sensation of flicker may still persist. He found, more- 
over, that when these surfaces are equally illuminated the sensation 
of flicker disappears. Upon these observations of Rood a photo- 
metric method has been built up by which hghts of the most 
different colors can be compared without any apparent difficulty to 
the eye, as far as color difference is concerned. 

Whitman + constructed one of the earliest flicker photometers 


* American Journal of Science, XLVI, September, 1893. 
+ Physical Review, III, p. 241. 


THE MEASUREMENT OF LIGHT 491 


and investigated the method. Whitman’s photometer is illustrated 
in Fig. 51. A card, as shown in the figure, having the radii IA 
and IB of 5 centimeters and 8 centimeters, respectively, was 
mounted on an axis I, which could be rotated, and which was 
affixed to a support moving on a photometer bar. The tube F so 
delimited the field of view that during one-half of the revolution 
only the portion CBD of the revolving disc could be seen, while 
during the other half of the revolution the card A was observed. 
With the disc rotating at proper speed, the photometer was moved 
along the bar until the sense of flicker ceased, at which point the 
brightnesses were assumed to be equal. Evidently, in an apparatus 
of this kind, if the speed of the disc is too high, the sense of flicker 
disappears without an equality of illumination, whereas if the speed 
is too low, the sense of flicker never disappears. It is important, 
_ therefore, that the disc should be rotated at the minimum speed at 
which the flicker can be made to disappear. With higher illumina- 
tions this speed will be higher than at lower illuminations, and with 
some one observer it may be somewhat higher than with another 
observer. 

Various other forms of flicker photometers have been produced 
and made commercially. One of the best known of these is the 
Simmance-Abady flicker photometer,* which is constructed as fol- 
lows: The photometric portion consists in a little wheel of plaster 
of Paris, the periphery of which is turned off in the form of trun- 
cated cones pointing in opposite directions (Fig. 52). The axes 
of the cones are eccentrically placed at equal distances on either 
side of the axis of the wheel and parallel to it. When this is 
placed with its axis of rotation parallel with the axis of the pho- 
tometer track, and is rotating, the eye sees the one conical surface 
illuminated by the one light and the other by the other light. In 
a certain position of the rotation only one surface is seen. In a 
position 180° from this only the other surface is seen. In the inter- 
mediate positions each surface> occupies a varying proportion of 
the field of view. If the rotation is carried on at proper speed 
a flickering sensation is perceived. ‘This little wheel is mounted 
on a carriage on which is also a clock-work suitable for producing 
the rotation. By means of a speed regulator, the apparatus may 
be adjusted to give the greatest degree of sensitiveness. 


* Proceedings of Physical Society of London, Vol. XIX: Philosophical 
Magazine, April, 1904. 


20* 


492 ILLUMINATING ENGINEERING 


The box carrying the clock-work and dise is, in one form of 
photometer, mounted on a horizontal axis so that the whole ar- 
rangement can be tilted and lights can be photometered at various 
vertical angles. A view of this photometer is given in Fig. 53. 
In the design of the instrument the various adjustments required 
have been provided for, especially the one necessary to secure the 
accurate alignment of the lights and of the photometer. 

A flicker photometer * is also made on the Martens system, in 
which the flicker is produced by the rotation of a combination of a 





Fig. 52.—Simmance-Abady Wheel. 


lens and a prism. This photometer is also arranged so that while 
the center portion of the field shows the illumination due to the 
right-hand light, the peripheral portion of the field shows the 
illumination due to the left-hand lght, and these conditions are 
rapidly interchanged. The photometer, therefore, may be used by 
observing the cessation of flicker in either half of the field or by 
observing the cessation of flicker by comparing one-half of the 
field with the other. The latter method evidently gives a higher 
sensibility. The optical system of this photometer is shown in 
Fig. 54, in which A is an opaque wedge, B the rotating system of 
lens and prism, and C the eye lens. 


* Zeitschrift fiir Instrumentenkunde, February, 1905. 


THE MEASUREMENT oF LIGHT 493 


The validity of the comparison between differently colored lights 
made by the flicker method has been seriously questioned, and has 
never been fully established. It is not definitely known at the 
present time whether the indications of the flicker photometer give 
a true measure of the equality of brightness or whether the cessa- 
tion of flicker indicates some other condition. Recent experiments 
made by Dr. Ives * have tended to answer the question to some 
extent. Dr. Ives has made Juminosity curves showing the com- 





Fig. 53.—Simmance-Abady Photometer. 


parison of spectral fields by the equality of brightness method and 
by the flicker method. The luminosity curves so obtained are 
quite similar in character. With bright fields, the luminosity 
curves obtained by both methods are practically coincident, indi- 
eating that where the fields are sufficiently bright, that is, where 
the Purkinje effect is relatively weak, the flicker photometer gives 
a comparison which is the same as the equality of brightness pho- 
tometer would give. When, however, the fields are dimmer the 
luminosity curves obtained by the equality of brightness method 
shift toward the blue end of the spectrum, which is the Purkinje 


eTrans, (2. S:, 1910: 


494 ILLUMINATING ENGINEERING 


effect, whereas, at the same time, the luminosity curves obtained 
by the flicker method shift toward the red end of the spectrum, 
which is a hitherto unobserved effect. Hence it is to be expected 
that at lower intensities the indications of the flicker photometer 
will differ from those of the equality of brightness photometer, 
and will differ in such a way as not only to nullify the Purkinje 
effect but even to reverse it. The flicker photometer, then, for low 
illuminations, must be considered as giving incorrect results, while 






MUM sn, 
HERA B 


| 


Fig. 54.—Marten’s Flicker Photometer. 





for high illuminations and for moderately large fields these results 
may be expected to agree with those of the equality of brightness 
photometer. 

It may be said, therefore, that while no difficulty is experienced 
with the flicker photometer due to color difference, yet the proper 
adjustment of speed is of considerable delicacy and requires a 
good deal of care, whereby the difficulty of the use of the instru- 
ment is somewhat increased, and, moreover, the use of the flicker 
photometer is quite fatiguing to the eye. 


THE MEASUREMENT OF LIGHT 495 


Color Measurements 


It is important not only to compare integral intensities of sources 
of light differing in color, but also to analyze the color differences 
and to give a numerical expression to the same. A rigorous analy- 
sis of the color differences between two sources of light may be 
made by the use of the spectrophotometer. The spectrophotometer 
is an instrument by which the spectrum of a standard source of 
hght and of an unknown source are produced side by side, and 
which has further an arrangement whereby the spectra may be 
brought to equal brightness in each successive color. For instance, 
the reds of the two sources of light are compared photometrically, 
afterwards the yellow portions of the spectrum are compared, then 
the greens, blues, etc. In this way the relative intensities of the 
various colors of the two lights are made known. Spectrophotome- 
try requires evidently not a standard of intensity but a standard 
of color, since the color of an unknown source is compared directly 
against a source of standard color. Various experimenters with 
the spectrophotometer have used different standards of color; for 
instance, the Carcel lamp, the Hefner lamp, the kerosene lamp, a 
gas flame, an acetylene flame, etc. Since these sources have them-. 
selves been intercompared spectrophotometrically, the various re- 
sults obtained are indirectly comparable. 

Practically all of the photometric arrangements which have been 
described for simple photometry are adaptable to spectrophotome- 
try. For instance, the law of varying distances may be used, or 
the rotating sector disc, or polarization apparatus, etc. 

One method which is very commonly used in spectrophotometry, 
and which is not ordinarily employed in simple photometry, is the 
use of a slit for varying the intensity. In any spectroscope a nar- 
row slit is placed in the focal plane of the collimator lens and 
limits the light received by this lens to a narrow line. Evidently 
by opening or closing the slit the amount of light admitted varies 
directly with the slit width. In many spectrophotometers the slits 
are fitted with micrometer screws by means of which the width can 
be determined with great accuracy. Moreover, the slits are of the 
symmetrical type; that is, instead of one jaw being fixed and the 
other movable, as in the ordinary slit, both move with the rotation 
of the screw in such a way that the center of the slit opening is 
unchanged in position. The validity of the assumption that the 


496 ILLUMINATING ENGINEERING 


brightness of the field is proportional to the width of the sht opening 
is not a correct one, as was first brought out by Murphy’s * experi- 
ments. Murphy compared the spectra of two incandescent lamps, 
first getting their relative intensities by adjustment of the slits 
and afterward interposing a rotating sector disc between one of 
the lights and the slit. The latter was then readjusted to bring 
again apparent equality in the field of the spectrophotometer. The 
open sectors of the rotating disc were 50 per cent of the circle, so 
that the light from that lamp was diminished to one-half of its 
former value. Consequently, the slit opening should have been 
always twice as great. 

By means of this arrangement it was shown that in the middle 
part of the spectrum the increase in intensity is less than the in- 
crease in the width of the slit, while at the ends of the spectrum 
the effect is reversed. Moreover, it was found that for slits nar- 
rowed below a certain point the intensity decreases more rapidly 
than the width of the slit. This results from losses due to dif- 
fraction, which are appreciable only when the slit is very narrow. 
The first effect can be explained by considering that the spectra 
in the field of a spectrophotometer are impure, and that in opening 
the jaws of a symmetrical slit a mixture of colors of varying lumi- 
nosity is obtained in the field of the telescope. If the rate of in- 
crease of luminosity is greater in the wave lengths at one side 
of the wave length corresponding to the center of the slit than is 
the rate of decrease on the other side, any increase in slit width will 
result in a correspondingly greater increase in luminous intensity 
of the light admitted, and vice versa. Hence it will be seen that 
error due to this cause will be reduced to zero in those parts of the 
spectrum where the luminosity curve for the source, as given by 
the prism of the spectrophotometer, is either straight or is so in- 
flected that the condition above is fulfilled. 

The author concluded “that the assumption that the spectrum 
intensity is proportional to the width of the slit is not strictly 
true,” and it is to be used with caution; that in the blue and cen- 
tral parts of the spectrum it is in error for slits in the ratio of 
1 to 2 as much as 2 or 3 per cent, while in the red this error may 
become as great as 10 per cent. 


* Astrophysical Journal, VI, p. 1. Also Capps, loc. cit., XI, p. 25. 


THE MEASUREMENT OF LIGHT 49% 


Spectrophotometers have been constructed in a variety of forms, 
and have been arranged to employ various means for the adjust- 
ment of the intensity. Some of the most important of these will 
now be described. 

Vierordt’s Spectrophotometer. This instrument is an adaptation 
of an ordinary spectroscope or spectrometer to spectrophotometric 
purposes. The slit of the spectroscope is removed, and for it is 
substituted a double symmetrical slit; that is, one divided into 
independent upper and lower halves, each controlled by a microme- 
ter screw. The light from the unknown source and from the 

































































Fig. 55 BEN hols: ivattie a actrophoroMerar. 


standard is brought into the respective halves of the slit by means 
of totally reflecting right-angle prisms placed immediately in front 
of the slits and facing in opposite directions. In this way the 
spectra of the two sources are seen-side by side in the field of the 
telescope. A vertical ocular sht is placed in the focal plane of 
the telescope so that only a short range of color is visible at any 
one time. Settings are made by opening or closing the slits until 
the fields are equal in brightness. 

Evidently two prisms may be used as indicated above on the 
ordinary slit of a spectrometer, but then the photometric settings 
must be made in some other way,.as, for instance, by varying the 


498 ILLUMINATING HNGINEERING 


distance of the lights or by the sector disc. An instrument of this 
character shows a line of separation between the two spectra, and 
on this account it is not easy to make accurate photometric meas- 
urements. 

Nichols’ Grating Spectrophotometer. In this instrument, which 
is illustrated in Fig. 55, a pair of symmetrical slits are used, but 
the slits are placed in a horizontal rather than a vertical position. 
A reflection grating is used for the production of the spectrum, the 
grating being placed on the end of an arm, pivoted close to the 


























grating and moving over a graduated sector. The telescope tube 
is pivoted on the same axis as the grating and moves also over 
the same sector. It is possible, therefore, to get the various colors 
of the spectrum by moving either the grating or the telescope. If 
the telescope and grating are moved together, the grating being 
kept normal to the axis of the telescope, the spectrum is a truly 
normal spectrum; that is, one in which the angular deviation is 
proportional to the wave length. With a Rowland grating, a much 
longer spectrum can be obtained than with the ordinary prism 
arrangement, whereby the errors due to slit width are diminished. 


THE MEASUREMENT oF LIGHT 499 


The normal spectrum has the advantage of giving much greater 
deviation in the red. The grating has, however, the disadvantage 
of giving a much feebler spectrum than a prism, so that the instru- 
ment can be used only with strong illuminations on the slit. 
Spectrophotometers with Lummer-Brodhun Cube. The Lummer- 
Brodhun cube has been adapted to spectrophotometry in a number 
of different instruments. In the Kriiss-Turnbull instrument the 
ordinary Lummer-Brodhun photometer has been. modified so that 
it can be used either as a simple photometer or as a spectrophotome- 
ter, the conversion from the one form to the other requiring only 





Fig. 57.—Krutss-Turnbull as Simple Photometer. 


a moment. A view of the instrument is given in Fig. 56, the 
- scheme of its optical arrangement when used as a simple photome- 
ter in Fig. 57, and a similar scheme of the spectrophotometer in 
Fig. 58. The photometer box is fitted with two collimators, to 
each of which is attached a prism which deviates the beam, leaving 
the collimator through an angle of 45°. The collimators, with 
their prisms, can be slid forward and back on ways attached to 
the side of the photometer box. For spectrophotometry it is nec- 
essary to add a direct-vision prism train for dispersing the light, 
and an eye-piece and ocular slit. When used for simple photometry 
these accessories are out of the way. The spectrophotometric set- 


500 ILLUMINATING ENGINEERING 


tings can be made either by moving the photometer along the track 
or by adjustment of the slits. The Lummer prism is ground so 
that the field, as seen, consists in horizontal bands which are 
illuminated by the two respective sources of light. When making 
observations the eye is accommodated on the cube and sees its sur- 
face illuminated with a given color of light. ‘The line of demarka- 
tion between the fields is a sharp one; consequently, settings can 
be made with high accuracy. 

In the Reichsanstalt ‘spectrophotometer, aes is illustrated in 


Fag | te 





Fie. 58.—Krtiss-Turnbull Spectrophotometer. 


Fig. 59, two collimators are used; placed at right angles to each 
other. The light from these collimators passes into the Lummer 
cube, and from the Lummer cube through the dispersing prism to ~ 
the telescope. With a proper eye-piece in the telescope, the eye is 
accommodated on the surface of the Lummer cube and sees the two 
fields with a sharp line of demarkation. Absorbing glasses are in- 
terposed so that contrast fields are produced and the sensitiveness - 
of the apparatus, is consequently very high. Settings may be made - | 


either by varying the slits or by a sector dise or some other ase es es 


method. 


THE MEASUREMENT OF LIGHT 501 


Brace Spectrophotometer. The Brace* spectrophotometer involves 
a special form of prism which serves the purpose of the Lummer- 
Brodhun cube and also of the dispersing prism. This prism is 
composed of two right-angle prisms of flint glass, ABC and ACD 
(Fig. 60), which when placed back to back constitute an equi- 
lateral prism. The back of ACD has a very thin band of chemically 
deposited silver SS across it horizontally. The prisms are pressed 
together with an intervening layer of a liquid of the same index 
of refraction-as the glass, so that they become a single, optically 
homogeneous prism, except for the silver strip.’ The paths of the 


































































































































































































































































































































































































































































































































































































Fic. 59.—Reichsanstalt Spectrophotometer. 


beams of light from two collimators TT’ furnished with symmet- 
rical slits are shown in Fig. 61. Beam b passes through the prism 
in the ordinary way and emerges dispersed into its spectrum, ex- 
cepting where it is interrupted by the band of silver. The portion 
of beam ec which is reflected by the silver emerges in a path parallel 
to that of b and is likewise dispersed. With the eye-piece of the 
telescope removed, and the eye fixed on the surface of separation © 
of the prisms, the observer sees three spectra, one above the other. 
The top and bottom ones are due to b and the middle one to ec. 
The line of demarkation is sharp, so that settings can be made 
with good precision. This instrument, however ingenious in theory, 


* Astrophysical Journal, XI, p. 6, 1900. 


502 ILLUMINATING ENGINEERING 


has, however, serious difficulties in its practical operation, one of 
which is that the halves of the prism become separated, due to 
the evaporation of the liquid by which they are united, and the 
rejoining of them requires special skill. 


G 
B D 


Fic. 60. 


Polarizing Spectrophotometers. Instruments using polarization. 
as a means for measuring the intensity of the light have been made 
by several physicists, among whom may be mentioned Glan, Crova 
and Konig, and which are fully described in the literature of the 
subject. Serious difficulty with the polarizing instruments is due 
to the great loss of ight in a polarizing apparatus, a loss which 





Fig. 61.—Brace Prism. 


becomes particularly important in the case of instruments using 
fluorspar. Fluorspar has been shown by Nichols to have a strong 
selective absorption in the blue. Since the blue end of the spec- 
trum is usually quite faint, it is particularly undesirable to have 
any of the blue rays removed when spectrophotometry is to be 
undertaken. 


THE MEASUREMENT OF LIGHT 503 


Expression of Spectrophotometric Results. Spectrophotometric 
results are usually given in the form of curves in which a hori- 
zontal 100 per cent line represents the standard color, and in which 
the color of the unknown lamp is represented in percentages for 
each of the various wave lengths. It is customary to consider the 
unknown source to have a value of 100 per cent in terms of the 
standard in the yellow at the D line, so that all curves cross at this 
point. With a whiter source of light than the standard, the curves 
then rise on the end towards the shorter wave lengths and fall 
below 100 per cent on the end of the longer wave lengths. The 
normal spectrum rather than the prismatic spectrum should be 
taken as standard in plotting spectrophotometric curves. 


Approximate Methods of Measurement 


Various methods for the evaluation of sources of light in respect 
to color which are shorter than the spectrophotometric method have 
been proposed. 

Two-Color Method. When we have to deal with illuminants, all 
of which have the same character of radiating surface, but which 
differ in temperature, the color of the hght may be expressed as 
the ratio of the intensity of the green portion of the spectrum to 
the intensity of the red portion of the spectrum as isolated by the 
use of green and red absorbing media. If, for instance, a carbon- 
filament incandescent lamp is burned at low voltage and is com- 
pared with a standard, holding a green glass between the eye and 
the photometer, a certain setting is obtained. Then a red glass 
is held between the eye and the photometer and another setting 


is obtained. The ratio o will give a constant which is 


f green 

red 
characteristic of the color of the light emitted by the lamp. If 
green 
re 
with the increasing whiteness of the light. This ratio, then, gives 
a factor by means of which the color composition is defined. ‘This 
has been made use of by DeLepinay and Nicati, and also by 
Weber, for facilitating heterochrome photometric measurements. 
The Weber photometer is fitted with a slide containing a green 
glass and a red glass which can be interposed between the eye and 
the photometer disc. In comparing lights of different colors, two 
settings are made; first, with the green and then with the red 


the voltage is raised on the lamp, the ratio of will increase 





504. ILLUMINATING ENGINEERING 


glass interposed. Evidently the color differences are extinguished 
by these screens, and excepting for the low intensities resultant 
upon the high absorption of the glasses, settings can be made 
without difficulty. The results are interpreted by a table given 


by Weber, which shows for each value of the ratio of ae the 


value of a corresponding constant by which the red reading must 
be multiplied to get the intensity of the unknown source of light. 





Fic. 62.—Ives’ Colorimeter. 


The values of the constant have been worked out by visual-acuity 
methods. Inasmuch as with the colored glasses interposed photo- 
metric settings are often quite uncertain, due to the dimness of the 
photometric field, and further, since the value of the constant 
with which the red setting must be multiplied to get the total 
intensity is subject to considerable uncertainty, due to the relative 
inaccuracy of visual-acuity methods, it is questionable whether 
this method of procedure really gives any more accurate results 


THE MEASUREMENT OF LIGHT 505 


than does a direct comparison of the photometric fields without 
any colored glass. Evidently this method does not apply to il- 
luminants which show a different type of selective radiation from 
that of the standard selected. 

Three-Color Method. As is well known, light of any tint may 
be produced by mingling in proper proportion light of the three 
primary colors, red, green and blue. This fact has been applied 
by Ives in an instrument for the measurement and definition of 
the color of light. This instrument is shown assembled in Fig. 
62 and in scheme in Fig. 63. It consists of a wooden box with 





Fig. 63.—Ives’ Colorimeter. 


an eye-piece at one end and with four adjustable slits at the other 
end. Through the slit D is admitted light which illuminates one- 
half of a photometric field. In front of the slits G, R and B are 
placed green, red and blue glasses. Through these slits comes 
the light from the other source, which illuminates the other half 
of the photometric field. The colored rays, green, red and blue, 
are mingled inside-the apparatus by a ring of lenses A, kept in 
rapid rotation by means of an electric motor. In the field of the 
instrument the observer sees one side illuminated by light passing 
through D, and the other side illuminated by the green, red and . 
blue rays which are recombined by the lens wheel. These slits, 
G, R, B, are adjustable in width by the levers seen in Fig. 62, 


506 ILLUMINATING ENGINEERING 


which move over divided scales. The slit D is also adjustable by 
another device. By varying the relative width of the slits G, R 
and B the tint seen on one side of the photometer field may be 
infinitely varied. In practice the instrument is first pointed at a 
brightly illuminated white surface which covers all four slits. 
The openings of the slits G, R and B are so adjusted that the 
color, as seen analyzed and recombined in the instrument, is the 
same as the color seen directly through D. This standardizes the 
instrument for white hght. Afterward, the illumination through 
G, R and B remaining the same, the unknown source of light is 
allowed to illuminate slit D. Then a readjustment is made of 
the slits G, R and B until the equality of tint is again observed. 
The ratios of the second settings of G, R and B to the first set- 
tings of G, R and B give the ratios of green, red and blue light in 
the unknown source as compared with the standard. Therefore, 
the composition of the light of the unknown source as compared 
with that of the standard is expressed in terms of the percentages 
of red, green and blue which it contains. 

The description here given of the operation of the instrument 
covers it only in its broadest features, and is not intended as a 
working instruction. The instrument is primarily intended for 
the measurement of tints of fabrics, pigments, solutions, etc., and 
not for photometric purposes. For photometric purposes, however, 
the instrument has considerable advantages. It is, in general, less 
troublesome to use than a spectrophotometer, and gives the results 
in simpler terms. In the case of sources of light which give line 
spectra, such as vacuum tubes and flaming arc lamps, the spectro- 
_ photometer can be used only with great difficulty, if at all, but the 
Ives colorimeter enables, in a case like this, a definition of the 
color to be given. 


Xx 


THE ARCHITECTURAL ASPECTS OF ILLUMINATING 
ENGINEERING 


By Water Coox 


CONTENTS 


The specialists in the construction of a building and the architect. Dif- 
ferences in the point of view. Identity of results desired by en- 
gineer and architect. 

Dangers of exaggeration on seth: of the facilities afforded by 
electricity. 

Three classes of buildings from the standpoint of illumination; day 
buildings, night buildings and day and night buildings. Units of 
illuminations. 

Various classes of buildings. Churches; theatsts art galleries, utilita- 
rian problems (special and general lighting). Domestic work. 
Hotels. 

The architect’s point of view. The relations of the architect and the 
illuminating engineer. | 


Every proper lecture begins with an aphorism, which is gener- 
ally a platitude; and this one shall be no grcenon to the rule. 

This is an age of specialists. 

At the bar we have criminal lawyers and cree lawyers and 
divorce lawyers. In the medical profession we have as many dif- 
ferent kinds of specialists as there are organs in the human body. 
And in building operations there are sanitary engineers, and heat- 
ing engineers, and last, but not least, illuminating engineers. I 
shudder to think that this universal trend of the times may event- 
ually affect even my own profession, if that evil day should come 
when it ceases to be in any sense of the word an art; and that we 
may have railway-station architects, and hotel architects, and 
architects who make a specialty of cosy corners. But this is an 
awful thought and has no relation whatever to what I have to 
say to-day. 

In the present condition of things we architects should be and 
are very thankful for the knowledge and the aid you bring us. We 
cannot, as a rule, pretend with any sincerity that.we can keep up 


508 ILLUMINATING ENGINEERING 


with the host of intricate details and new devices, changing, as they 
do, from day to day, which enter into the construction of a build- 
ing. And we turn to you, with your special scientific knowledge 
of them, to advise us and to help us in a spirit of confidence and 
good-will. 

But this age of specialists has, I will not say its disadvantageous, 
but sometimes its trying side. If you have a headache, you may 
predict with some certainty that the oculist will find beyond perad- 
venture that it is due to some disorder of the eye, and will prescribe 
a new pair of spectacles; the nerve doctor, after a careful investi- 
gation, declares that it is simply a case of neurotic disorder; and 
- the stomach man assures you that if you refrain carefully from 
eating anything you are fond of all will be well. | 

Something similar to this often occurs in a building. The heat- 
ing man looks upon it as so much space to be heated and ventilated, 
and from his point of view it is successful or not according as these 
important results are properly reached. His illuminating brother 
conceives that to be able to see clearly by artificial light is the most 
vital need of the human race. And the irrational architect, who 
realizes these needs as well as anyone, but who has above all tried 
to design something which shall be convenient and appropriate 
and beautiful, in his hours of discouragement sometimes fancies 
that all these experts are a flock of vultures, each seeking what 
part of his creation he may pounce upon and devour. It is because 
of these moments of unreasonable irritation—when we long for the 
days when the architect could design a building where there was 
no plumbing, no steam heat, and where lamps and candles supplied 
all the light attainable—that the unfortunate fable of the enmity 
between the engineer and the architect arose. I am glad to know 
that this is really only a fable; in our saner moments we embrace 
each other as we should and swear eternal brotherhood. | 

For, after all, we are really both seeking exactly the same things, 
and with the same zeal. We both wish to have our buildings so 
lit that those who make use of them shall be able to see sufficiently 
and agreeably. We both wish to have this illumination enhance 
rather than detract from whatever architectural effects or beauties 
may exist; and above all that the men and women—the living 
pictures for which our buildings are in a way but the frames— 
may look as well as possible and not as ill as possible. Now, these 
conditions are not always easy to conciliate; and as architecture 


ARCHITECTURAL ASPECTS OF ILLUMINATING ENGINEERING 509 


itself has often been very aptly defined as a series of compromises, 
so this is especially true of the problems of illumination. 

No solution which ignores any one of these different conditions 
is, except accidentally, entirely successful. A scheme which is 
quite perfect in so far as the provision of light for its various uses 
is concerned, may be absolutely hideous in its effect upon the archi- 
tecture or upon the people; and one which sets off these to the best 
possible advantage may be so unfortunate from the standpoint of 
utility as to be an utter failure. It is our common task,to seek 
how we may conciliate, these various points of view in our work. 

The tasks which we have presented to us have been rendered at 
once easier and harder by the infinite possibilities of electricity. 
It is in a way entirely delightful to be such masters of the situa- 
tion, and to realize how few limitations are imposed upon us. We 
can, when the question of expense is not an all-important one, 
do almost anything we please. We can put all the lights we wish 
anywhere we please; we can in an instant extinguish a quarter of 
them or a half of them or all of them; we can vary the intensity 
and the color of our light at will. But, on the other hand, this 
unlimited freedom which we enjoy is a perilous gift. How can we 
be at all sure of choosing the best amidst such a host of alterna- 
tives? 'The temptation to excess of various kinds—in the amount 
of light furnished or in the effects produced—is ever present. 
Many of us can remember with what joy we welcomed the electric 
lighting of our streets. It filled us with an almost childish delight ; 
we were to be able to walk about without fear by night as by day. 
And then one day the first electric sign appeared; and to-day the 
“Great White Way ” has become the synonym of the world’s great- 
est vulgarity. Not only is it by night filled with dazzling and 
glaring caricatures—whiskey bottles, petticoats, automobiles and 
corsets—all whisking about and revolving and flashing various 
colors at the same time; but in the day-time the frame-works which 
serve for these spectacular advertisements cover up whatever the 
buildings may have to recommend them, and rear their hideous 
shapes into the heavens above. 

“I believe no other country suffers from this eyesore in the same 
degree as our own; and I confess to the greatest surprise that no 
one, so far as I know, has suggested taxing them out of existence 
or forbidding their use entirely. Even if the electric signs are 
wanting, the temptation to illuminate the whole exterior of a 


510 ILLUMINATING ENGINEERING 
& 

building is sometimes yielded to. I have seen somewhere a photo- 
graph, taken at night, of some building in the west, where all the 
vertical and horizontal lines were so many strings of electric lights. 
Every detail of the architecture disappeared and there remained 
nothing but a ghastly incandescent skeleton, endurable if it were 
but for a single night to celebrate some great event, but horrible 
to think of as a permanency. All this would seem to be one more 
evidence as to how powerless aesthetic considerations are in our 
land; and it all happens because electricity is so easy. 

Now this same easiness in obtaining results is with us in all 
our work. And the desire to produce the new and the startling 
is continually tempting us towards excesses of various sorts. It 
is, of course, true that in lighting, as in all other architectural 
matters, a strong distinction can be made between those places 
which are lived in continuously and those which are only visited 
for a comparatively short time.. But, even in those of the latter 
class—amusement resorts of various kinds—we often see effects 
in the lighting which, while they arouse our admiration by their 
cleverness, fill us with a certain apprehension as to what may de- 
velop in the future. The means at our command are so infinite 
in their possibilities that the danger is not to find a striking and 
effective scheme, but to avoid finding one that is too striking and 
too effective. It is sometimes almost a piece of good luck that the 
inevitable question of cost arises and checks our unholy desires. 

As all Gaul, according to Caesar, was divided into three parts, 
so there are from our present standpoint three kinds of buildings— 
those which are used only in the day-time, those which are used 
only in the night, whether it be a real or an artificial night, and 
those which are used both by day and night. 

When a building is only used in the day-time, the questions of 
artificial light hardly exist as important factors. | 

When a building is used only at night, then the illumination 
becomes one of its most important elements; and nothing in its 
interior should be designed by the architect without keeping this 
constantly before him. Every proportion, every detail, every bit 
of ornamentation is changed and modified by the arrangements 
of the lighting—arrangements which must often depend much 
more on utilitarian considerations than on anything else. In all 
this class of buildings, then, it seems to me necessary that from 
the very beginning the question of illumination should be con- 


ARCHITECTURAL Asprots or InLuMINATING ENGINEERING 511 
& 

sidered. It may easily change or at least modify the whole design. 
And so in these cases I hope the illuminating expert will come very 
early upon the field. It is also especially the case in such buildings 
that the lights do not simply serve to show off the architecture and 
the decoration; they are in themselves decorative motives of the 
highest importance and value, and must be so considered. 

When a building is used both by night and by day the questions 
become somewhat more complicated. It is almost inevitable that 
the lighting by day has the first place. The architect designs his 
rooms from that point of view, taking into account the natural 
light of day in everything that he does, and striving to adapt his 
architecture to it. In all such cases, then, the general conditions 
of artificial hghting would seem at first sight to be rather simple 
than otherwise. We have under consideration a room designed for 
~ daylight. What is more rational than to see what can be done to 
produce with the means at our command the same quantity of 
hght and the same effects or some approach to them, at night? 
Now, there are a good many cases where this is a correct standpoint, 

and I shal] mention some of them later on. But the principle is 
far from being a universal one; indeed, in perhaps the majority 
of cases it is incorrect. The use of the same rooms is very fre- 
quently quite different at day and at night; and a decided variety 
in effect is not only permissible but desirable. Then the lights 
involve fixtures, which are often very important and valuable ele- 
ments of decoration in themselves when they are not lighted, and 
almost controlling factors when they are. And, finally, it is only 
exceptionally the case that we are able really to reproduce the light 
of day, or anything very close to it, even if we desire to do so, 
and we are apt in the endeavor to fall between two stools. -For 
we have neither the sun nor the sky; and in our endeavor to ob- 
tain them we miss the opportunities for a decorative twinkle and 
brillianey of the lights, apparent, even though they may be shaded 
so as to avoid any painful glare and sparkle. And so I think we 
should hesitate a long time before considering the various schemes 
of concealed lights, or illumination by reflection only, unless the 
conditions seem to demand them imperatively. 

In those buildings, then, which are used equally by day and by 
night, while I am far from saying that the illumination should not 
be considered at an early stage in any design, it is yet not quite 
so important a determining factor of the architecture itself as in 


512 ILLUMINATING ENGINEERING 


those used only by night. None the less it should be kept in mind, 
and the placing of the lights, their shading, the direction and 
power of the light rays should all be so calculated so as to set off 
the prominent features of the rooms to their best advantage, as 
well as to fulfil those material conditions which, do what we will, 
are always with us. 

Perhaps this is as good a place as any to call your attention to 
a question which is, I believe, not a new one. Is it not probable 
that we should generally improve our effects if the light unit—in 
electric work generally the 16-candle-power lamp—were reduced? 
In other words, should we not be better off with four lights or with 
six lights where we now use two—the total amount of light being 
the same? A good many years ago I was fortunate enough to be 
present at an official ball given in what was considered the finest 
ball-room in the world, I mean that of the old Hétel de Ville in 
Paris. The great room—this was almost in ancient days—was 
ht by 11,000 wax candles; and the effect has remained in my 
memory as the most beautiful in its own way I have ever seen— 
altogether fairy-like, and immensely becoming to the elaborate 
toilets of the women and the dress uniforms of a good many of 
the men. Now, I suppose, no light has ever been used which is 
quite as charming as candle-light; but some approach to it can 
perhaps be obtained in electric illumination if we use small units 
and plenty of them. I think this matter is very worthy of your 
careful consideration. 

But it is in order now to say a few words about the various 
structures in which illumination is an important factor; and there 
are very few in which it is not so. And, first, let us consider the 
question of church lighting. 7 | 

What is a church? Remember, said my revered and beloved 
teacher in Paris, that a church is first of all a place for religious 
processions. Well, nous avons changé tout cela, and I imagine 
this definition would hardly be agreed to in this country, where 
the nearest approach that we have to religious processions (at least 
in our Protestant churches) are wedding processions. It is true 
that our Catholic churches are still temples for great religious 
ceremonies, and in a lesser degree this is true of the Episcopal 
houses of worship. 

But those of most other denominations are really religious lecture 
rooms. It is one of the extraordinary anomalies of our time and 


ARCHITECTURAL ASPECTS OF ILLUMINATING ENGINEERING 513 


‘country that in a great majority of cases these sects insist, from 
some sentiment difficult to explain, upon following in their church 
architecture those same mediaeval types which their forefathers 
attacked and demolished with fury as savoring of idolatry, and 
reminding them of that Babylonian lady whom I will not further 
-particularize. 

If, then, we have to deal with a Catholic church, built on medi- 
aeval lines, we can simply follow the tradition; a splendid blaze at 
the altar, a minimum of lighting in the chapels, and practically no 
light at all for the worshippers; the body of the church in that 
mysterious twilight for which the churches of old’ were designed. 
And this is entirely natural and permissible when the services are 
‘such as to require no reading on the part of the people. It avoids 
in any way calling away attention from the actual ceremonies 
which are taking place, to the worshippers in the body of the church. 
Any considerable further illumination must detract from a beauty 
which is largely dependent on the lights and shadows of the archi- 
tecture itself, seen somewhat dimly at all times by the hght of 
the stained-glass windows, or by that cast upon it from the lights 
at the altar. But what are we to do with a Gothic building in 
which there is no altar, in which the services are distinctly untra- 
ditional and unecclesiastical, with the practical requirement that 
each member of the congregation must have light to read by? I 
do not think this problem has ever been really solved in such a 
way that the architecture shall be helped or at least not injured, 
and that the lighting shall form an appropriate decoration to it. 
lt has been tried in a good many different ways—clusters of lights 
around the columns, hanging lamps and various other devices; I 
do not recall at present any instance where each pew had its 
discreetly shaded lamps, which would seem to me an experiment 
worth trying, and at all events promising less harm than the others ; 
but I venture to doubt whether even you gentlemen will ever suc- 
ceed in satisfying yourselves or ourselves in such a case, and I 
ean only hope that in time other views as to the church archi- 
tecture of the dissenting Protestant churches may prevail, and 
that the problem may become an easier one. 

As we all know, there are already a great many church buildings 
which are distinctly unmediaeval and which -seek quite other solu- 
tions. The various types of the Renaissance have been taken as 
models, with varying degrees of success—sometimes very marked 


514 ILLUMINATING ENGINEERING 


success ; and with somewhat of the same difficulties of illumination 
as exist in the Gothic types, although in a less marked degree. 
And, finally, there are some few examples where all tradition has 
been cast to the winds and an honest attempt has been made to 
produce a religious lecture-room—call it a religious theater if 
you will. The great, the almost unsurmountable obstacle encoun- 
tered is the difficulty of giving to these rooms that indefinable 
character which we recognize as religious, when all accepted types 
are discarded. If complete success on these lines is ever reached, 
then the problems of an illumination which shall also reflect the 
religious sentiment will be an important and a difficult one; but 
until such is the case we cannot discuss even in the most general 
way the means to be employed. 

I began this somewhat unprofitable disquisition by asking “ What 
is a church?” and now I must ask “ What is a theater?” A month 
or so ago I went to a representation of the Meistersinger in the 
Prinz Regenten heater in Munich. The performance began at 
four o’clock in the afternoon and was finished somewhat after nine. 
After each act there was an entr’acte, the lesser ones lasting about 
a half an hour, the longest something over an hour. Everyone in 
the auditorium went out, as a matter of course, during each en- 
tr’acte and walked in the foyers or in the large and beautiful gar- 
den attached to the theater and forming a part of it; and during 
the longest entr’acte the entire audience took supper in the restau- 
rant, which was also a part of the theater. So that the whole 
entertainment was a social gathering of which the opera itself was 
the all-important part. Accordingly, the auditorium of the theater 
was simply an auditorium—a place to hear the opera. It was 
designed and decorated with the greatest sobriety and simplicity, 
and was hardly lighted at all—just enough to facilitate the in- 
comings and outgoings. As a result the stage itself was a blaze 
of glory, not only in its lighting but also in its contrasting richness 
of color and decoration. 

Perhaps an opera house on these lines may some day be possible 
and be asked for here. But at present the auditorium of a theater— 
I mean of a large and important theater—is not only a place to 
see the spectacle, but is also a place to exhibit the public to each 
other in the entr’actes to the best advantage, a place where the 
finest gowns and the most resplendent jewels are set forth to the 
admiring gaze of the world at large. Given these conditions and 


ARCHITECTURAL ASPECTS. OF ILLUMINATING ENGINEERING 515 


the lighting of the auditorium becomes of the first importance, and 
has nothing whatever to do with what happens upon the stage. A 
very considerable lighting from the ceiling seems imperative. . For- 
tunately, we are enabled now to do away with the great. central 
chandelier, which was formerly inevitable, and distribute our lights 
while keeping them high up. And each box, or each group of 
people, must also be lit, not only that they may see but that they 
may be seen. In one of the large Paris theaters, the Chatelet, I 
believe, a good many years ago, indeed, before the age of electric 
lighting, the ceiling was constructed nearly entirely of glass, and 
above this were placed very abundant lights. JI think the actual 
light was sufficient in quantity for everybody to see easily enough 
to read the programme or the evening paper. But after a while the 
scheme was abandoned—not, I think, for any other reason than 
that brilliancy of effect which seemed then as now an absolute 
necessity was so wanting. 

It seems to me that the plan often followed of placing lights 
around the galleries, on the outside of them, is not a successful 
one. Lights so placed are not agreeable to the people who sit 
behind and a little above them, and, moreover, they tend to em- 
phasize the comparative darkness of the boxes, which ought to be 
so illuminated as to set off to the best advantage the living pictures 
which occupy them. However, if we accept the general conditions 
which I have outlined, and the results to be sought for, the prob- 
lem does not seem to me to be one of unusual difficulty. It is at 
least a thing to be devoutly thankful for that we can turn all our 
lights off at a moment’s notice, and for a time forget the public 
and remember only the play. 

It is only in comparatively modern times that the artificial 
lighting of an art gallery—rooms for the exhibition of paintings 
and sculpture—has become such an important one. Even to-day 
few, if any, of the great historical collections of Europe are shown 
at all in the night-time. But in this busy country, perhaps more 
people visit an exhibition of pictures or of sculpture in the evening 
than at any other time; and it seems to me that if I were a . 
painter, exhibiting my pictures in one of our galleries, I should 
find it very hard not to be torn by conflicting emotions, and not 
to be continually asking myself whether. I should paint for day 
or for night. It is evident that the problem, whether of daylight 
or of night-time, is here a purely utilitarian one. Here, at least, 


516 ILLUMINATING HNGINEERING 


we have only one thing to consider, and that is that the works of 
art shall look their best. We must of necessity ignore to a great 
extent the architecture of the room and the appearance of the 
people in it. The pictures, and the pictures alone, are to be 
considered. : 

Now, it is universally agreed that in the day-time the only ad- 
missible and the only possible light is a top light. The questions 
which arise in regard to it are questions of detail—how large the 
ceiling light shall be, how the rays of light from above shall fall 
upon it, and similar matters. And so in artificial light the aim 
should be to reproduce as far as possible the same conditions ex- 
actly, taking particular care that the evening light should not be 
an exaggeration, should not give to the pictures an artificial and — 
meretricious brillianey which they possess at no other time, and 
for which they are not or at least should not be painted. 

The most ideal solution that I am acquainted with, although I 
am informed that it has been copied and even improved upon else- 
where, is in the Albright Art Gallery in Buffalo. In this gallery 
there is a very large glass-ceiling light, through which in the day- 
time the light is admitted by skylights. In the night there is a 
very powerful electric lighting above this ceiling lhght, arranged 
with reflectors in such a way as to reproduce the directions of the 
daylight rays. The glass so softens and diffuses the electric light 
that the effect is astonishingly similar to daylight, and is alto- 
gether charming and without exaggeration. There is one disad- 
vantage; in the day-time the shadows of the lighting apparatus 
are cast upon the glass of the ceilmg. This is not very conspicuous, 
it is true, and could, I think, have been avoided; indeed, it may 
have already been rectified by this time. 

I suppose that this admirable result is only obtained by a very 
great expenditure of power, and that our thanks for it are due 
not only to the skill which elaborated the scheme but also to 
Niagara Falls, which furnishes the wherewithal. 

But even if the general scheme should prove impracticable in 
some cases, it still seems to me that similar, if perhaps less perfect, 
results could be obtained by means of lamps and reflectors care- 
fully chosen ; and that it is possible to light our pictures sufficiently 
and agreeably without casting upon them that satanic glare which 
we are too accustomed to; in other words, an illumination which 
in the direction, the intensity and the brilliancy of the light rays 
shall in a great measure counterfeit the light of day. 


ARCHITECTURAL ASPECTS OF ILLUMINATING ENGINEERING 517 


When the exposition is one of sculpture I think the question is 
a quite different one. While some of the sculptors may not agree 
with me, I do not favor a diffused light from above for their works. 
And in this regard, I am happy to quote from a work which is in 
some sort the bible of the architects, the “ Elements and Theory 
of Architecture,” by M. Guadet. “I recommend,” he says, “in 
the case of paintings a diffused light that I warn you against for . 
works of sculpture. With these the necessities of lighting are 
different. Sculpture receives its effects from light directed in a 
certain and specific direction. Paintings and drawings have their 
effects in themselves. Light models sculpture, it should only il- 
luminate painting.” Many of you have seen and remember the 
Venus de Milo in the Louvre, alone in a square room of rather 
modest dimensions, lit by one large window on the right hand of 
the statue. It seems to me that there can be no question that this 
masterpiece of antiquity is far more effectively displayed than 
would have been the case if it were set under a skylight. The 
ideal rooms to show statues would be a long gallery, not too wide, 
lighted from one side only, and if possible divided into compart- 
ments opening widely into each other. In that case any artificial 
lighting would naturally follow the lines of the daylight in direc- 
tion and intensity. 

And I may here say that what is true of works of sculpture is 
true also of people. These also “receive their effects from hght 
directed in a certain and specific direction.” So that when the 
appearance of the people in any room is to be taken into important 
consideration, we must remember the sacrifices we are making as 
well as the possible advantages we may obtain by treating the room 
as a picture gallery. 

The consideration of the lighting of large assembly rooms, thee 
I mean, of a utilitarian character, such as the halls for our legis- 
lative bodies, is somewhat similar to that of picture galleries. For 
in these rooms a day-lighting by skylights and ceiling lights is 
almost inevitable. We must remember that they are often crowded 
with people, and that what is to be sought for is a general illumina- 
tion permitting everyone in the room to see easily, even as they 
move around, and that what is to be avoided is any glare and 
sparkle which may tire and even injure the eyes. So that in these 
cases, also, a secondary light from above reproducing the day effects 
would seem to be the best attainable and, on the whole, a satisfac- 
tory solution. 


518 | ILLUMINATING ENGINEERING 


There is a very large class of buildings of an even more strictly 
utilitarian character, such as offices, factories and commercial build- 
ings, all of which have to be hghted; some of them, in these days 
of unreasonably tall buildings or of unreasonably narrow streets, 
by day as well as by night. And whatever can be done to render 
the life of the thousands of toilers in them less unendurable is a 
real boon to our suffering humanity. It does not come within my 
province to discuss these in any detail. But it is my experience 
that one general question is apt to present itself in nearly all of 
them—the choice between a general illumination of the working 
space, and a special illumination for each person or group of per- 
sons. I have been fortunate enough to have been associated with 
one of the members of the Society of Illuminating Engineers and 
one of my fellow-lecturers in the lighting of a number of the branch 
buildings of the New York Public Library, erected by the gener- 
osity of Mr. Carnegie in that city; and the evolution of ideas as 
one after another of these buildings was erected is not without in- 
terest. It should be said that they consist entirely of a number of 
public reading-rooms, and of certain offices in connection with these. 
These reading-rooms have high book-shelves against the walls, and 
a certain number of lower free-standing book-shelves on the floor; 
but the main space is given up to tables and readers. When the 
first of these buildings was put up, the accepted idea was to pro- 
vide sufficient general illumination for everyone in these reading- 
rooms to see well to read or to select the books from the shelves ; 
and this was pretty successfully accomplished by usual methods. 
But when the monthly bills were presented there was a cry of 
anguish ; and the lighting was reduced, with the result that no one 
was satisfied. To make a long story short, the final result has been 
that in the later library buildings the general illumination is com- 
paratively small, care being taken that it is so arranged as to light 
the book-cases on the walls sufficiently ; that the independent and 
low book-cases are lighted only when the need arises, by swinging 
brackets which are lit automatically, and that the position of each 
table for readers is carefully established and the table provided 
with a lamp of sufficient power, very carefully calculated and 
shaded for the readers around it. This seems to be the best, as it 
is the latest solution of the problem; but there is always a fly in 
the amber, and when the librarian of a given library changes, as 
sometimes happens, the new incumbent usually wishes to change 


ARCHITECTURAL ASPECTS OF ILLUMINATING ENGINEERING 519 


the position of some of the tables, and is met by a non possumus 
which occasions some weeping and gnashing of teeth. The origi- 
nally planned arrangement is very nearly an inflexible and un- 
elastic one. But I am glad to testify here to the skill and the 
patience of the illuminating engineer in solving for us this some- 
what complicated task. If some one of you should ever invent a 
thoroughly practical storage battery for our lamps the last objec- 
tion would be removed. 

In a very large number of cases this or some similar scheme of 
what I may call special illumination is the most feasible. But 
there are occasions where a general illumination is of absolute 
necessity. When this happens, the principal difficulty is to avoid 
a glitter and a glare which may, while giving sufficient light, prove 
tiring and injurious to the eyes, even when the lamps are hung 
above the line of vision. This it has been sought to avoid by sus- 
pending lamps from the ceiling which are entirely concealed from 
below, but which by means of powerful reflectors throw their light 
upon the ceiling, which, serving in its turn as a reflector, diffuses 
the light through the room. I am not sufficiently informed as to 
the success of this system to speak with any authority; it would 
seem as if the loss of light by these successive reflections must be 
considerable, but this may not be true. Very favorable opinions 
as to its efficacy I have heard expressed; and we await the final 
verdict of the scientific world with much interest. Even in certain 
cases where utility is not the only consideration, there are possi- 
bilities in this idea. I can imagine a room designed for one reason 
or another in a severely classical style—Greek, or something like 
it—where the effect might be much better on the architecture itself 
than any system of apparent lights, which could hardly fail to be 
at least an incongruous element. But in any room where it is used 
and considered as a part of an architectural scheme, it follows of 
necessity that the treatment of the ceilings becomes exceedingly 
important, for these are the most strongly lighted of anything in 
it, and becomes naturally the most important features. The colors 
used also would need the most careful consideration from the stand- 
point of light, both those of the ceilings and of the walls; but it is 
quite unnecessary to insist further on these points. 

The various questions which arise in the lighting of domestic 
work are those with which the architect has perhaps most to do, 
and the engineer least. None the less they interest both of us. In 


520 i ILLUMINATING ENGINEERING 


such work the element of woman enters very prominently, properly 
and inevitably. And even when the sun of equal suffrage for the 
sexes has risen far above the horizon over which it is now peeping, 
it is entirely probable that there will still exist some differences 
between men and women, even in their predilections concerning 
lighting. It is a general experience that a strong light from the 
ceiling is not beloved of the fair sex. Sometimes it is somewhat 
unkindly alleged that is because such lighting is not considered 
becoming to their toilets or their persons, but if such were indeed 
the case all of my bald-headed male contemporaries would share 
their views, and this I have not found to be the case. Then most 
women dread a glare of light more than their husbands or their 
sons do; and this has to be taken into account, especially in the 
matter of placing the lights so that they can be easily shaded. 
In domestic work a great variation at different times in the lighting 
of the same rooms is most desirable. It is for this reason, I fancy, 
that any electric or gas lighting is almost always supplemented by 
lamps and candles in rooms which are lived in. These can be 
placed in 20 different positions, and help us enormously to avoid 
a certain formality imposed by the conditions at present existing. 
And here again we long for the ideal storage battery, so that we 
need not continually be tripping up on cords, or cutting holes in © 
our carpets, as we do with our present arrangements of floor and 
base plugs. And, finally, the fixtures and the lights in them are 
nowhere of greater importance as architectural motives, not even 
in the stateliest of halls. 

Of course, there are certain large mansions which have their ball- 
rooms, and what we may call their state apartments, where the 
conditions do not vary materially from public places of assembly. 
Here we can afford to be gay and glittering; certainly the strains 
of a Strauss waltz or of the Merry Widow do not accord with a 
gentle and general diffused illumination which suggests work rather 
than play. But even in such rooms it seems wise to make a certain 
distinction, as they are after all a part of the house; and above all 
to avoid an ostentation which may easily turn to vulgarity; and 
which may hurt that idea of the home which we are always dwelling 
upon and claiming somewhat absurdly as a special possession of 
the English-speaking races. uly 

As different as possible from domestic work is that in the modern 
_ hotel, with all its appurtenances, and in buildings of a similar 


a . 


ARCHITECTURAL ASPECTS OF ILLUMINATING ENGINEERING 521 


character. The large and important hotel of our day and genera- 
tion, and especially of our country, has not, it is true, ceased to 
be a lodging-house, but it is very far removed from the lodging- 
house of our ancestors. Its prominent attractions now, heralded 
far and near, are its tea-rooms, its ball-rooms, its banqueting halls, 
its palm-rooms, its Flemish tap-rooms and its roof-gardens. And 
in all these, where there is an ever-shifting public, bent on amuse- 
ment alone, and eager for new and startling impressions and effects, 
it is quite in order that in some measure restraint should be cast 
to the winds, and the unexpected should be sought for. The whole 
thing is somewhat in the nature of a vast spree, and there is a 
certain diabolical pleasure in entering into the spirit of the thing. 
We have in this country an astonishing lack of opportunity of en- 
tertaining ourselves simply out of doors. In all European cities 
we can dine, if the weather permits, in gardens under the shade of 
trees; we can listen to music, we can enjoy the fresh air or watch 
the passersby. If we take our coffee afterwards; if we indulge, as 
the whole world does nowadays, in afternoon tea, we get out of 
the house if we can, we sit on the broad sidewalks, if there is 
nothing better, or we go into some park; for the idea that nothing 
should be allowed in a park except walking around or sitting on 
benches or amateur love-making does not occur to anybody. But 
here quite different ideas prevail. In the City of New York, which 
our recent census informs us has a population more than equal to 
that of Paris and Berlin combined, there are actually but two or 
three places where one can have any of these simple indulgences, 
and these are for the rich only, and not for modest engineers and 
architects. We must perforce go indoors and make the best of it. 
And so it is quite natural that the hotels should endeavor to profit 
by this state of things by introducing into their entertainment 
rooms lattices and vines and trees, more or less genuine, and trying 
to make us think for an instant that we are indeed out of doors; 
or by turning their roofs into the semblance of a real garden when 
they can, with babbling brooks and waterfalls and beds of flowers. 
It is impossible not to admire the skill with which this is some- 
times accomplished ; and in all these schemes the most potent single 
instrument is the electric light, which is here forced to do a hundred 
different things and produce a hundred different effects; for in 
nearly all cases our garden effects are garden effects by night. 
The grotesque .and the weird appear not infrequently, but I am 


522 ILLUMINATING ENGINEERING 


tempted rather to say a few words about some examples which 
seem to me to be unusually successful and not to exceed the just 
limitations of such work; I mean certain of the rooms in the Hotel 
Astor in New York. 

The tea-room is a large rectangular room, entirely unlighted by 
natural light. The whole room is a rather simple version of out 
of doors, with a liberal use of lattice-work, a latticed ceiling largely 
of glass, through which is admitted a subdued bluish moonlight, 
shining down through hanging vines, which themselves carry a 
number of glimmering lights, half hidden by the leaves. The 
statue of a nymph has moonlight cast upon it. The illumination 
is very moderate and very agreeable; on each table there is a lamp 
fixture, but the lamp itself, turned downward, is entirely concealed 
by an opaque tulip-shaped shade. However, you cannot but re- 
mark what exquisite complexions the ladies all have; and it is not 
till you find out that the lower part of the glass in each lamp is 
of a delicate pinkish color that casts an almost imperceptible glow 
over the neighboring objects that you offer up thanks for an art that 
is at once so effective and so discreet. 

The large banqueting or ball-room and a very successful gallery 
in the hotel have glass ceilings partially covered with various pat- 
terns of woodwork, in some cases of extreme elaboration. And here 
also similar bluish moonlight effects are produced, varying in in- 
tensity according to the needs of the occasion. And these rooms 
have in addition hanging fixtures and some wall illumination dif- 
fering entirely in color and quality. The contrasts in these cases 
seem to me very entertaining. 

And, finally, the roof-garden, with its rock effects and water- 
falls and flower beds and statues has some charming effects of 
lighting, very appropriate to just such a place, which must be seen 
to be appreciated. 

I have discoursed at some length on this phase of our subject, 
and on this special example, because here the designer has been 
in a way forced to consider the decorative and artistic possibilities 
at the same time with the actual material needs. It seems to me 
that often he is apt to divide his work too strictly into two classes— 
the utilitarian, where nothing else is considered ; or the ornamental, 
where there are no limitations, and he can do his best—or his worst. 
But, really, there is no problem where he would not do well to 
consider both sides, and to ignore neither. The most practical one 


ARCHITECTURAL ASPECTS OF ILLUMINATING ENGINEERING 523 


can generally be solved satisfactorily without an absolute disregard 
of appearances. 

If, in these show-rooms of the modern hotel, where one is to be 
congratulated on the attainment of unusual, even startling effects, 
all these devices are quite legitimate and praiseworthy, there is all 
the more reason why by contrast many of the other rooms of a 
more modest character should have a studiously restrained and 
modest lighting. I think that in such rooms, those, for instance, 
where one likes to go for a quiet half hour, it often happens that 
we are too much in the lime-light, and long for something ap- 
proaching domesticity. This same fault obtains in many of our 
clubs, where a social sitting-room is often as brilliantly lighted 
as if a ball were in progress. Now we go there to meet our friends, 
to chat, often to rest from more serious pursuits; and are con- 
demned to pursue this quiet kind of happiness in a blaze of glory. 
I was interested to notice in the quieter parts of the Hotel Astor, 
where the lighting was accomplished in a conventional way by 
hanging ceiling lights enclosed in obscured glass globes, that in 
every case the top of the light inside the globe was shaded, so as 
to avoid the strong rays on the ceiling. 

In these very desultory observations on the Architectural Aspects 
of Illuminating Engineering, I have made no attempt to speak 
to you of the technical side of the science; on the contrary, I have 
prudently and carefully avoided it. You will doubtless consider 
it quite anomalous to hear a discussion of any phases of the subject 
in hand without a single mention of watt consumption or of 
tungsten lamps. But my object has been to give you some idea of 
the point of view of the architect as distinguished from that of the 
engineer; and to show you that, in the mind of the former, illumi- 
nation is only partially a separate and distinct problem from all 
the others with which he is confronted, and in many ways very 
similar to them ; that it is one which he considers in the same way 
and endeavors to solve by the same processes of thought. I have 
endeavored to impress upon you, that while the question of suffi- 
cient light is always the most important one, there are yet other 
aspects of the subject which must be taken into consideration in 
order to achieve the best kind of success; that the lighting and 
the architecture can never be considered separately and without 
reference to each other, but must be studied together, as inter- 
dependent subjects, and that while sometimes the lighting must 


524 ILLUMINATING ENGINEERING 


yield something to the architecture, it may well happen that the 
architecture must be modified to suit the conditions of the il- 
lumination. 

Accordingly, I have confined myself mostly to stating the con- 
ditions of the various problems which arise, from an architect’s 
point of view, and have only suggested the solution; for these vary 
in every particular case, and no hard and fast rules can be laid 
down. And I realize very fully that even in these statements of 
conditions, I can only speak for myself, and that others of my 
professional brethren may see things in a quite different light. 
Yet I am convinced that our general way of looking at things will 
not differ very widely; and I shall be well satisfied if this point of 
view has been made any clearer to you by what I have said to-day. 

There remains to be said one word about the relations of the 
architect and the illuminating engineer, and those of both of 
them to the building, and that word is—a sincere co-operation. 
The architect conceives that he has a right to be consulted in 
everything that concerns his creation. He has many unhappy 
moments when he has to look upon the paper that his clients put 
upon the walls of his rooms and the furniture they put in the 
rooms themselves; all of which has often been chosen according 
to the advice of the enlightened salesman, without the slightest 
conception that such matters might interest the man who has de- 
signed the building. Let him, at least, be spared the pain of 
having a lighting scheme forced upon him without an opportunity 
of consulting and being consulted. Let us do all in our power that 
we may meet together at an early date to discuss the questions 
which interest us both equally; that we may exchange our views, 
that we may be prepared to give and take mutually, to the end that 
we may part as we begin to-day—the best of friends. 















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